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why does the water level in a fish tank go down?

Why Does the Water Level in a Fish Tank Go Down?

If you are the proud owner of a tropical fish tank, you will have noticed how much the water level in your tank will go down.

The water level in your fish tank is always changing. This is due to evaporation, and affects tanks of all shapes and sizes. The warmer water temperature in a tropical tank accelerates the evaporation process.

In this article, I will discuss what evaporation is all about (science kind of stuff), how much you should be topping your fish tank up, and a few ways how to slow down the process in your tank.

why does the water level in my fish tank go down?
I’m sure there was more water in here yesterday?

How High Should the Water Level be in a Fish Tank?

Your fish tank should be filled to within an inch of the top rim of the tank.

I always see photos in online groups of lovely tanks but the tank is only filled to three quarters with water! The more water for the fish, the better, as it gives them more room to move around. Anything more than an inch in your tank that isn’t filled with water is just a waste, as there is no use for it.

Your tank will sound noisier with the water level being lower, as when the water goes back into the tank after going through the filter, you will hear it trickling in. Depending on how many of the little outlets you have on your filter, it can be quite irritating.

When I very first started out keeping fish, I didn’t have my water topped up as much as it should have been. I think I had a space of about 2 inches. I spent half the time panicking as I could hear water trickling and thought something was leaking somewhere, and the other half of the time wanting to go for a wee due to the constant sound of water!

fish tank water level
This tank needs topping up.

Why Does the Water Level in my Fish Tank Go Down?

Have you ever gone on holiday for a few days, and come back to a lot less water in your tank? It can be quite a shock if you haven’t seen it for a few days as it can reduce by quite a few inches.

Now, unless you have a leak somewhere (a puddle on the floor would be a dead giveaway here), the water level going down is normal, and happens with every tropical fish tank, regardless of the size.

The water level reducing is due to evaporation.

Now it is time for the science stuff…

What is Evaporation?

Evaporation is a physical process where water is transformed from a liquid to a vapour.

According to The National Geographic’s website, evaporation happens on a global scale, and accounts for 90% of the moisture in our atmosphere.

In the earth’s water cycle, water from lakes, rivers and oceans evaporates to the atmosphere, and then comes back down again as rain (or snow).

In the case of our fish tanks, which are miniscule in size compared to the examples above, water is lost to the air in the room.

It doesn’t come back as a mini shower while you’re watching TV, so you have to replenish the water in the tank.

Evaporation occurs only at the surface of the water, so the bigger the surface area, the more evaporation will occur.

Water molecules are always in constant collision with each other. The rate of collision is increased with an increase in temperature.

The temperature of the water is the leading factor that controls the evaporation rate, which is why the water level in your tropical tank goes down quite quickly, compared to a cold water fish tank.

The water loss occurs when its molecule gains enough energy to break away from the liquid phase and rise into the air as a vapour phase.

Water Movement.

Wave-like ripples on the water surface will increase the rate of evaporation. This is because the water molecules will get even warmer by the increase of bumping into other water molecules (the constant collision mentioned earlier). The increase in temperature speed up the evaporation process.

fish tank water level too low
This tank needs to be topped up at least another 2 inches.

How Do I Slow Down my Aquarium Water Evaporating?

Things like air pumps will speed up the evaporation process as it increases the movement in the water.

If you don’t have a lid on your tank, you should get a glass lid. Having that inch free in between the surface of the water and the fish tank lid will help to slow the process down as it acts as a barrier. Some of the water that otherwise would have gone into the air will turn into condensation on the underside of your lid and will drip back into the tank.

Don’t have your tank lights on for too long. You should already know this from a welfare for your fish point of view, but it will also help to slow down the process a little.

Make sure your tank is not in front of a window. Again, this is something you should already know not to do.

You could turn down the temperature in your tank slightly, but only if it is still within a comfortable range for the species of fish you have.

Make sure you are regularly topping your water up as the level reduces.

Remember I mentioned earlier on about how the water coming back out of the filter can be heard going back into the tank if your water level is too low? If you can hear the water splashing back into the tank, it is a clear sign that the water movement is increased because of this. Increased water movement speeds up the process, so once you start losing water, it will reduce at a faster rate unless you top it back up again.

What to do when Water Evaporates from a Fish Tank.

When the water starts evaporating from your fish tank, you need to keep topping it up. It will be a never ending process, but it is a necessary one, as reduced water in your tank, means reduced oxygen for your fish, and less space for them to swim in.

Make sure when topping the water back up that you have added any required water conditioner to that water first, and that the water is not too cold.

Add the water slowly so as not to create so much water movement that your fish think it’s a tidal wave.

If you carry out a weekly partial water change, topping the water up will be second nature as you just incorporate it into that regular task.


The water level in your tank reducing is normal and natural. It happens in every single tropical fish tank.

Keep an eye out to ensure the water level doesn’t go down too much, and top your water up regularly and you won’t any issues with it.

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