Holding the line... Simple thoughts on evaporation and strategies to counter it.

One of the things I've been obsessive about with my botanical-style aquariums is water exchanges. To me, the one single most important thing we can do as aquarists is to exchange water. And of course, the whole idea of water exchanges is to create stability in your aquarium's environment. This is pretty much "Aquarium Keeping 101", and we all know this, right?

And then there is what happens between water exchanges: Evaporation...and how we manage it...

Evaporation is a pretty big deal. And it has immediate impact on the environmental stability of our aquariums. Dissolved solids, minerals, organics, and salt ( when present) do not evaporate. When evaporation occurs in your aquarium, the concentration of substances in the water actually increases as the water volume decreases. 

This is important to many fishes, which require stable environmental conditions in order to achieve optimal health.

In marine and brackish water aquariums, the specific gravity of the water can increase significantly as a result of evaporation, if fresh water is not added in an equal volume to replace it. This obviously has health implications for the animals which reside in the aquarium.


Of course, in our botanical-style, blackwater aquariums, evaporation concentrates substances dissolved in the water, changing the environmental parameters over time. Now, we could argue, with our emphasis on experimentation and recreating the shifting water levels of say, African forest streams, rain puddles, and vernal pools, or Brazilian igarape, that water depth varies, and organic substances in the water concentrate, and that this is something our fishes can tolerate.

However, in my opinion, this would be a weaker argument for a closed system aquarium, because there is simply not the volume of flow-through, or even nutrient export processes occurring in our tanks that there happens in Nature, even in all but the tiniest, most stagnant bodies of water (yeah, this little puddles where you find annual killies or wild bettas come to mind).

Now, these gross water-level changes typically occur over longer periods of time in natural systems than they do in the confines of a small aquarium. Rain, atmospheric conditions, runoff, and other phenomenon affect this. And in this instance, the words "tolerate" versus "thrive" are sort of at odds with each other, I think. 

All that stuff being equal, the one thing that I am a big believer in with every aquarium that I keep is environmental stability.

Not the mindset of "pegging the pH at 6.3 without any fluctuation", mind you- No, rather, I proffer a stability within a small range. With evaporation, the "range" can become a lot broader, and the fluctuations can happen a lot faster than we'd like. In our aquariums with concentrations of botanicals, the ratio of" pH-reducing/organic input-capable materials" to water obviously increases as the water level decreases.

It's not one of those, "Ohmigod, my tank is going to crash if I don't do something about this right now!" sort of things, but dealing with regular evaporation in the botanical (or brackish) aquarium is an important consideration in the context of environmental stability. Stress from constant environmental fluctuation is a longer-term thing with fishes, yet it can lead to very tangible health issues over time if not addressed.

How much a given aquarium evaporates is based on a myriad of factors, such as the ambient humidity/temperature of the room it's kept in, the time of the year, how wide of an opening the tank has, etc., etc. There is no real "standard formula" of how much a given aquarium will evaporate in a specified amount of time. I've had 300 gallon aquariums that lost 4-5 gallons a week to evaporation, and much smaller tanks that lost that much in a day!

Obviously, in smaller aquariums, the affects of evaporation are more impactful and serious, and some means to address the issue should be considered above and beyond the routine weekly water exchanges.

The easiest way to deal with evaporation is to simply add more water (fresh water in the brackish or marine tank, as the salt concentration will increase as water evaporates). "Well, NO SHIT, SCOTT!"

I'm a freaking genius, I know.

Kind of common sense, but something to think about, right? I'd go so far as to say that some regular "top-off" with freshwater is absolutely vital for the brackish tank, and fairly important for the lower pH, botanical/blackwater aquarium. And of course, we'll no doubt have many heated discussion on the merits of using "pre-tinted" tipoff water versus simply pure RO/DI water in botanical/blackwater tanks..

Keeping track of how much to add isn't particularly difficult, either, as you might guess. Liek, seriously low tech.

You can simply mark the side of your aquarium with a line in an inconspicuous place with permanent marker, and make sure that the water level never decreases below the line during normal operations. This simple and crude visual gives you a decent guide as to how much your water is evaporating on a regular basis. You can even get fancy and use the old standby math formula to determine how many gallons a given measure of water level loss represents (Sorry, metric users, I don't have the exact numbers for the conversion at the top of my head, but it's easy enough to do):

Multiply length by width by height of the tank and divide by 231.

Thusly, if you have a tank that's 48"x14"x20", the product is 13,440. Divide this number by 231 and you get 58.18 gallons. So, if you lost, say, 1/2" inch of height in the water column due to evaporation, that works out to 48x14x.5 = 336. Divide by 231 and you get 1.45 gallons. So...one half of an inch of water loss is equivalent to about 1.5 gallons of water.

Fairly significant, right?

I promise never to demonstrate math again in this blog. I think that is literally the only "formula" I've ever memorized (used to concoct "fantasy fish tanks" in my head for decades!), and I know some math whiz out there is going to be like, "Um, excuuuuuse meee- there is a better way to do this..." So forgive my remedial math skills!

But you get the idea, right?

The simplest way to combat the evaporation issue in the aquarium is to add a little water every day to "hold the line" in your tank. The visual marker makes it easy. However, this simple methodology only works if you're around to do it. Go away on vacation for a week or two, and you're unattended tank will definitely fall behind. Now, will this spell disaster? Likely not, unless you have an overflow weir in the aquarium or rely on a specific water level to keep pumps and heater submerged (and if there is little room for evaporation in this regard).

Yet, again, it's about stability. It's just a "thing" I have. Anything I can do to keep stability in my systems is a good practice. Good habits are always nice to acquire.

Today's ridiculously simple thought, with important impacts.

Stay observant. Stay curious. Stay proactive. Stay engaged...

And Stay Wet.


Scott Fellman

Tannin Aquatics 


Scott Fellman
Scott Fellman


3 Responses

Joshua E Morgan
Joshua E Morgan

October 22, 2020

Cool! Just be forewarned that chocolate gouramies are – from what I’ve heard from those who have kept them – surprisingly hostile to each other for their size and are not great nano fishes…they really need a 20 long and a group of at least 6 gouramies to avoid stressing each other out so much from their arguments that they all fall (fatally, in many cases) ill. I never ran into similar issues with my licorice gouramies, though – I’ve had several male Parosphromenus ‘sentang’ claiming territories in a 5 gallon before (granted, it was a father and his sons, but still…little to no violence).

Scott Fellman
Scott Fellman

October 21, 2020

Hi Joshua,

Great idea, actually! I’ve been toying with the idea of Chocolate Gouramis in a nicely heated nano tank I have, but it’s open top, and I know that the idea of maintaining humidity in the atmosphere above the tank is beneficial, so…who knows, lol!


Joshua E Morgan
Joshua E Morgan

October 20, 2020

I’ve almost always had smallish tanks, and these have generally had plastic wrap as a lid component to hinder evaporation (with a few small holes for ventilation, of course). It also halts small fish like wild bettas and killifishes from jumping (don’t expect it to stop a 5-6 inch Betta pi or a largish cichlid, though).

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