Osmosis in Planted Aquariums

by Jul 8, 202218 comments

Osmosis is the process in which any solvent moves through a membrane in a direction from lower concentration to higher concentration that tends to equalize the solutes’ concentration on both sides. The membrane should be semi-permeable and good examples are the cells of the aquatic faunas and plants.

So why we should know about Osmosis and how it relates to our planted aquariums and successfully keeping it? In this article, we will discuss all about Osmosis in Planted Aquariums.

Table of Contents

Osmosis in Planted Aquariums?
Hypotonic Solution?
Hypertonic Solution?
What are the Effects in Floras and Faunas?
Isotonic Solution?
How Does Osmosis Relate to the Quality of Water We are Using?
Safe Range
TDS is not a Worthless Measurement

Osmosis in Planted Aquariums?

So let me repeat again: Osmosis is the process in which any solvent (water) moves through a semi-permeable membrane (fish or plant cells) in a direction from lower concentration to higher concentration that tends to equalize the solutes’ (salts, ions, sugars, dissolved solids, etc.) concentration on both sides.

This is due to osmotic pressure and is used by every living thing to regulate its cell solute concentrations within safe levels (osmoregulation). Osmotic pressure is spontaneous and cells doesn’t need to expend energy.

Without osmoregulation, it changes the fish’s blood pH, digestive tract, and immune system, greatly affecting its health and risk of catching diseases or worse death.

Without osmoregulation, plants’ water and nutrients uptake, photosynthesis, and general stability will be affected (resulting in melting or wilting). Within the plant, Osmosis ensures that all cells and structures have the correct water pressure and volume. This is also the reason that plants remain upright.

Guppy Close up

For example, if you put a freshwater fish in pure water free of any minerals and other salts like distilled water, this means that the fish cells have more solutes than the water outside it. This is called a Hypotonic Solution.

When this occurs, more water will enter the cell than leave it to balance out the concentration of solutes on both sides.

The water forces its way into the fish due to osmotic pressure (lower concentrations to higher concentrations) and quickly dissolves the solutes in the fish’s body. The fish cell will continually swell, and under such conditions, the fish cell may burst = dead fish. 

Schooling Fish
Dutch Style Planted Aquarium Aquascaped by Omar Krishnan Afuang Philippines

Dutch Style Planted Aquarium Aquascaped by Omar Krishnan Afuang Philippines

So what if the freshwater fish is put in water with very high solutes than the water in its cells (in the case of salt water or water with very high dissolved solutes – TDS/Hardness/Salts)? The fish will continually lose its water within its body (water goes from lower concentrations to high concentrations).

This is called a Hypertonic Solution, the opposite of the Hypotonic solution above. In this case, more water leaves the cell than enters it.

As a result of this, the osmotic pressure is too much, and the fish could not adapt faster than it is losing water until its cells shriveled = another dead fish.

Another example: this is why slugs and snails shrivel and die when you sprinkle salt on them. The water in their cells wants to equalize with the high concentration of salt outside the cells.

Jungle Style Planted Aquarium

Jungle Style Planted Aquarium. Image by zoosnow from Pixabay

Floras and Faunas in our aquariums are affected by Osmosis differently. The floras and faunas can tolerate different solutions.

For example, in the Hypotonic solution mentioned above, fish cells will burst because of too much water entering. However, in a plant, plant cells need more water than animal cells.

The plant cells have more solute than the water outside so water rushes in. The plant cell could burst, but thanks to a rigid wall surrounding the cells, it will not. The cells expand until it pushes against their cell wall, creating pressure.

No more water can enter and this is the main line of support why terrestrial and aquatic plants remain upright for non-woody stems. A hypotonic solution is ideal for plant cells.

How about the Hypertonic solution for plants’ and animals’ cells? Water will rush out of both plant and fish cells and the cells will shrivel: for fish = dead; for plant = since there is no pressure anymore and less water = wilt.

So what if we have equal amount of solutes in the cell and the water outside? This is called an Isotonic solution.

Dutch Style Planted Aquarium Aquascaped by Ayong Go Philippines

Dutch Style Planted Aquarium Aquascaped by Ayong Go Philippines

An Isotonic solution is ideal for animals including our aquatic faunas. But the ideal balance is impossible to achieve in our planted aquariums. Not to mention how to determine the ideal balance.

How do you determine the amount of solutes in plants and fish cells and compare it to your aquarium water? Yeah, good luck with that.

How about an isotonic solution in plants? The plant cells are no longer full of water or not turgid anymore, with not enough pressure to make them stiff and upright. The plant’s leaves will droop as well.

So what are we implying here? How does Osmosis relate to the quality of water we used in our planted aquariums?

As it turns out, plants cannot thrive in spotless water due to the absence of the nutrients needed to create their own food.

Aquatic faunas and floras don’t only need pure H2O molecules. They also need salts (KH, carbonates), nutrients (Ammonium, NPK, dissolved CO2 for plants), and minerals (calcium, magnesium, iron, etc.) present in the water but also not too much.

Crystal Red Shrimps

Crystal Red Shrimps

Using a zero KH-GH-TDS water source in a bare aquarium or container is like the hypotonic scenario above for our faunas. Actually, zero final TDS water in a planted aquarium is pretty much impossible.

Anything that you add to your planted aquarium from the substrate, hardscape, fertilizers, plants, water conditioners, fish food, dead organic matter, dissolved solids, CO2 injection, etc. will add to your TDS.

So we are safe on this low extreme end. We will recommend at the very least maintaining 80 TDS after your planted aquarium has been established.

Glowlight Tetra in a Planted Aquarium

Glowlight Tetra in a Planted Aquarium. Image by Cody Wu from Pixabay

You can still go lower up to 20 ppm TDS if not impossible in a planted aquarium, but there is no point lowering to that as it takes a lot of effort, OCD to kick in, and headaches. You will not be able to enjoy your planted aquarium that way.

On the other extreme end, using a very high KH-GH-TDS water source is like the Hypertonic scenario, very high and with unknown solutes compared to what our plants and fish are used to. As an example, using super hard deep-well water (GH in excess of 530 ppm).

Or a very high baseline TDS water source in excess of 500, etc. Take note, that 500 TDS is just the baseline TDS of the water source. It will still increase once you use it with your planted aquarium.

Overstocking, overfeeding, tank neglect, and a filter that cannot keep up with the load can also lead to water with very high dissolved solids (TDS).

Planted Aquarium Aquascaped by Brylle Isaac Philippines

Planted Aquarium Aquascaped by Brylle Isaac Philippines

So to summarize, in an established planted aquarium, we will recommend maintaining your water from 80 TDS on the lower end up to 300 TDS on the higher end to save you from all the headaches and anxieties. This is after you set up and added everything to your planted aquarium.

Personally, I keep my planted aquarium from 120 to 130 ppm TDS after and before water change respectively. Very narrow, but it is manageable because I am using RO/DI water with remineralization. The baseline TDS of my RO/DI nowadays is 26 to 27 ppm. I know what I put in my tank. 

Nature Aquarium Aquascaped by Bart Capundag Philippines

Nature Aquarium Aquascaped by Bart Capundag Philippines

It is the number that I settled with that works for my plants and fish. Every planted tank is unique and everyone should come up with a number that works for their setup. So we recommended a safe, manageable, and feasible range above.

You may also realize that TDS is not really a worthless measurement to take with regard to planted aquariums. It is one of the most important water parameters. It will affect the osmoregulation in your faunas and floras cells.

It can affect their health, immune system, ability to purge waste from their body, nutrient and water intake, stability, etc. 

Many issues with faunas that get attributed to large pH swings (you mess with your KH), or water hardness are actually caused by Osmotic shock. 

This is also true if you don’t drip acclimate your newly bought fish and didn’t research their preferred water parameters. It can cause the melting or wilting of aquatic plants too. 

For your reference, the terms mentioned here might be unfamiliar to some:

For KH and GH, go here.

For TDS, please go here.

For safe water sources that we can use in our planted aquariums, go here.

For the Drip Acclimation Method, go here.


In this article, we discussed what Osmosis is and its relation to the quality of water we are using in our planted aquariums.

It can affect our faunas and floras’ well-being and we also recommended a safe and manageable range of TDS that we can maintain.

Want to Explore More?

Nature Style Aquarium

It is mostly considered to be one of the most popular aquascape designs today. And for an excellent reason, this style is simply breathtaking and very natural looking. We are replicating nature, after all.

Nature-style aquascape uses natural wood branches or driftwood, rocks, and stones to project a natural theme and provide a sense of flow to the form and composition.

The Fastest Method – How to Cycle a Planted Aquarium

The fastest method to cycle your tank is to use your old filter media, gravel, soil, hardscapes, and even part of the old water from your old aquarium to your newly planted aquarium.

But this needs careful planning and execution for it to be successful.

Planted Aquarium Tank Dimensions

After choosing the ideal location of your aquarium at your home and the stand to be used, you have to determine your planted aquarium tank dimensions. You have to take measurements of the Length, Width, and Height (LxWxH) of the stand. Take into consideration where you will put your equipment, tools such as aquarium filter, aquascaping tools (straight tweezers, curved scissors), siphon, etc.

Nutrients – Planted Aquarium Water Parameters

Our aquarium is very much like a septic tank for our faunas (fish/es, snails, and shrimps). Whenever they excrete, or you are overfeeding, and those organics started to decay, or when dead plant matter decays, Ammonia is produced. When faunas die and decay, they will also produce Ammonia, lots of it actually, until they are removed. This causes ammonia levels in our tank to spike.

HOB Filters – Types of Planted Aquarium Filters

They are also called hang-on-back filters (HOB), and are designed to hang on the back of your aquarium, eing! Power filters are the most commonly used planted aquarium filter because they provide good to excellent mechanical and biological filtration simultaneously. They can also provide the needed water surface agitation for aerating your water.

Paludarium Ideas

A Paludarium is a type of vivarium that contains water and land in the same environment or encasement. The design can simulate natural habitats such as rainforests, jungles, streams, riverbanks, and bogs. In a Paludarium, part of the aquarium is underwater, and part is above water.


  1. Angee

    I would really love to have an aquarium at my house one day. It is interesting to know about this. I have booked your website for future references.
    We written article thank you.

    • Lemuel Sacop

      Hello Angee,

      Thank you for visiting my website and I am glad that you have plans already to have an aquarium at your home one day. Obviously, I would recommend a planted aquarium. I can’t wait to hear about your experiences and what aquascape you can come up with.

      Let me know if you need help, please don’t hesitate to contact me.

  2. Virendra

    I always love going to the aquaria in the area here, and because of this I wanted to know more about aquaria and fish in general. Which brought me to your website! Thank you so much for such an extensive explanation about why it’s so important to get the water in your aquarium exactly right! I didn’t have a single clue that minerals and pressure is so important for your fish too! Can you give any advice for beginners on what is a good fish and aquarium for ultimate beginners? That would be zo amazing! thank you

  3. Kat

    Thank you so much for this informational post! I think about getting an aquarium, and as I notice reading your articles, it’s much more to it then buying an aquarium, and put some plants, fishes and water in it … I will get through your articles one by one. Maybe you have a suggestion where to start if someone doesn’t have any experience? Which topic is the most important?
    Have a great day!

  4. Chas

    Hi Lemuel,
    Thank you for helping me understand why the fish in my earlier aquariums were dying. I was simply because I did not have enough education on Aquariums. Now, with your website i have a wonderful one stop resource for anything I need to know in the future.
    Is their a rule of thumb percentage between plant population and fish to help keep things in balance? Thank you for this great site.

    • Lemuel Sacop

      Hello Chas,

      Thank you for coming back. As for your questions, there is no rule in plant population, you can either plant sparsely or go all out. It depends on the design you have in mind. Here is a quick link to all the ways you can design your planted aquarium and the selection of plants and faunas that go with them:


      For faunas stocking, it depends on the volume of your tank, what fish you plan to keep, plus the adult size of that fish. There are actually 2 rules, the 1 inch per gallon rule and the surface area rule but both have room for errors. For example, the 1-inch per gallon rule doesn’t take into consideration if you have an oversized filter that can keep up with the waste load and you don’t take it literally.

      For example, if you have a 10 gallons tank, and you did add substrate, hardscape, some plants, etc. you don’t have 10 gallons of water anymore, as anything you added that has volume will subtract to your water volume. It is more like minus 10 to 15 % of 10 gallons. You don’t stock that 10 gallons with x10 1-inch size comet goldfish literally because they can grow big over time, fast (we don’t really recommend goldfish in a planted aquarium anyway). There is a lot of nano fish in the fish trade that looks a lot like goldfish for alternatives. Bigger fish produces more waste.

      But if you stock that 10 gallons with x10 Ember Tetras, that is manageable, because they stay small (about 0.5 inch) and eat less (do not overfeed). Better stay with nano fish/es in small planted aquariums. But if you have a filter that can keep up with the Bioload in a 10 gallons tank, like using x10 turnover rate = 100 gph filter, you can add another 10 Ember Tetras.

      There is a lot of schooling/shoaling nano fish for planted aquariums, and mostly you need at least 6 of them to school or shoal. So keep that in mind as well as they will get stressed if you don’t keep the minimum number.

      Another example, in a group of six Discus, you need at least 55 gallons and above. A small Arowana can easily outgrow a 50 gallons tank, you need at least 100 gallons for one.

      Personally right now, I have about 15 Tetras, 1 Platty, 1 Otocinclus, 6 SAEs, about 12 Amano Shrimps, about 15 Horned nerite snails, and a few pond snails, in my 35 gallons planted aquarium. But I have an x10 gph Canister filter, heavily planted.

      In my 20 gallons-filled Riparium, I have Guppies continuously reproducing that I cannot keep count. I only have a 7 watts submersible pump pointed upwards, like a fountain, to provide lots of surface agitation to oxygenate the water, a lot of lava rocks for beneficial bacteria colonization, and a lot of submerged and marginal plants to aid with cleaning the water.

  5. Christine Duts

    I’ve always known the term osmosis but never really understood what it was. Now I realize how important this is to keep a healthy environment in our aquariums. As a child I had an aquarium and our fish did very well in there, so I think we did it right 😉 but some fish are more delicate than others, isn’t it?
    This explanation opened my eyes. It is extremely important to ensure that the fish you have in your aquarium have an ideal water environment. How can I know, though, how much TDS there is? Are there specialized measurement or detection tools for that?

  6. Zvezdan

    Excellent article and very informative on how to properly maintain the balance of fresh and saltwater for fish and their survival and also interesting and well written for snails 🙂 thank you for this informative article and I’m sure this information will be useful to everyone. I wish you good health.

    • Lemuel Sacop

      Hello Zvezdan,

      Thank you for appreciating this article and visiting my website. I hope, in some way or two, I was able to help you in your decision to keep a planted aquarium at home. I can’t wait to hear about your experiences and what design you can come up with.

  7. Safia

    Hi Lemuel,

    As I understand this process, it is important that we first look at what osmosis is and how it works. Osmosis occurs when a solvent moves across a selectively permeable membrane from an area of low solute concentration to high solute concentration until there are equal concentrations on both sides. The dissolved molecules move through the membrane because they are attracted by either higher or lower water potential on the other side.

    This explanation opened my eyes. It is extremely important to ensure that the fish you have in your aquarium have an ideal water environment. How can I know, though, how much TDS there is? Are there specialized measurement or detection tools for that?

    Hope find answers for my questions.

    Take care.

  8. Nina

    Thank you for sharing this article. It was quite explicit and informative. I didn’t realize an aquarium had such a fragile composition. I have read other explanations that made it seem quite simple to create a healthy aquarium environment. After reading your article, I will have to reconsider how to create the perfect aquatic environment. This article will be bookmarked as a reference for me.


    • Lemuel Sacop

      Hello Nina,

      Thank you for coming back. Just don’t linger beyond the extremes of what we recommended (80 to 300 TDS – this range is very manageable and safe), and don’t use subpar water quality in our aquariums just like we don’t like drinking subpar water. We are actually protected on the lower extreme of the range because in an established planted aquarium, 0 to 20 TDS is very hard to maintain, if not impossible. Zero TDS is impossible once you added everything in your tank. Everything that you add to your planted tank adds to your TDS.

      We should only worry on the higher end; just don’t use a water source with a baseline of >500 TDS to save you from all the headaches. Or dilute it with a very low TDS water source like RO/DI or distilled water.

  9. Cynthia

    This is a fascinating article. I have always been interested in aquariums and would have loved to try it with live plants. Unfortunately, I have never had any luck with keeping fish alive so I gave it up years ago. I am going to assume that the imbalance in the water was the culprit and I thank you for the information. I think I’m ready to try again…with the help of this website!

    • Lemuel Sacop

      Hello Cynthia,

      Thank you for visiting my website and appreciating this article. I hope in some way or two, I was able to help you in your decision to keep a planted aquarium in your very own home. I can’t wait to hear about your experiences and what design you can come up with.

      If you have more questions or needed help, don’t hesitate to contact us.


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