Evaporation, stability, and that stuff we all do...

If you've kept aquariums for any length of time, one of the things that you've noticed is that they a re subject to evaporation. Particularly, open-top aquariums. 

Why should we care? Well, for one thing, dissolved solids, minerals, organics, and salt( when presnet) do not evaporate. When evaporation occurs in your aquarium, the concentration of substances in the water actually increases as the water volume decreases. 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 has health implications for the animals which reside in the aquarium.

Now, even in our botanical, 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 occurring in our tanks that there happens in nature, even in all but the finest, most stagnant bodies of water (yeah, this little puddles where you find annual killies or wild bettas come to mind). And 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. And "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 "pegging the pH at 6.3 without fluctuation", mind you- No, rather, 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 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 a 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 others 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). 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..

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.

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 is going to be like, "Um, excuuuuuse meee- there is a better way to do this..." So forgive my C minus 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.

Or, you could automate it.

Yeah, I know- ME of all people- recommending automation. Scary.

I utilize a very simple, surprisingly reliable and efficient automated tipoff system. The one I use is the "Smart ATO Micro" system. It consists of a small LED optical sensor to detect water level (which can work in complete darkness), and a tiny little pump that can move water up to 6 feet. There are no moving parts to fail, which is a huge plus in this category of equipment. It's stunningly simple to use- in fact, it literally took me- "Mr. Mechanically Challenged", like 5 minutes or so to set up! You simply locate the pump in a reservoir (which could be a small aquarium, big water jug, or other container used to hold your top-off water), attach a length of tubing (included) of sufficient length to reach your tank, and attach the tiny optical sensor via its magnet mount at the height you want the water level to remain at.

When the water level goes below the sensor height, the top-off pump is activated, and the micro-sized pump will dispense the exact amount needed to bring the water level back to the pre-determined level. Since it's so accurate, you'll see and hear the little pump going on multiple times throughout the day, simply topping off milliliters of water at a time to keep the level constant. About the only maintenance you need to do is to keep the sensor clean (wipe it down once in a while with a toothbrush or similar), and make sure that you have sufficient water in your "reservoir" to assure top-off. 

This little thing has changed my life, in terms of hassle, with the office blackwater/botanical aquarium you see on these pages. It's made the maintenance of constant water level a real snap, especially with our busy work/travel schedule and weekends and such, and I can't recommend it enough! In fact, after over a year of testing, we've decided to carry this little system on the Tannin Aquatics website! Look for it soon! Once we get more into the "estuary" stuff, you'll see that an auto-topoff system is just going be considered "standard equipment" for our purposes!

Anyways, this article was not intended to be a less-than-slickly-disguised ad for a soon-to-be-added product. (LOL, really!).  There are a number of other excellent auto-topoff systems on the market, including the Tunze "Osmolator", which does an equally outstanding job, but is a bit more complex. I like "simple", and for our purposes, the unit we elected works great! I'm always looking for anything that can create stability in my system, and having a strategy (even if it IS just adding some fresh water to your tank every day) is one of the easier practices that you can engage in to accomplish this.

So, we can look at evaporation as a problem, an annoyance, or simply part of the natural  world- something that we need to deal with to keep our systems running smoothly. It's just another aspect of aquarium keeping that seems like everyone knows about, but it's not typically addressed in the context of the aquariums that our community works with, so I thought it worthy of discussion!

Stay cautious. Stay consistent. Stay open-minded.

And Stay Wet.


Scott Fellman

Tannin Aquatics.



Scott Fellman
Scott Fellman


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