Water Movement

I think that I can safely say that the movement of water in an aquarium is one of the most disregarded, yet important, aspects of successful aquaria. If you haven’t ever really given it any direct thought before now, you might be wondering what I’m talking about. Let’s see if I can help you understand its importance, and then we’ll talk about some ways that you can create healthy movement of the water in your fish tank.

The movement of water performs several vital functions. First of all, it prevents thermal layering. Ever been swimming in a lake? Notice how cold it gets a few feet under the surface? That’s because hot water rises and the colder water sinks to the bottom due to convection currents. With current, you can mix the water so that there are no cold or warmer layers.

Second, water movement provides the fishes with a current in which to swim. Before I began keeping African Cichlids, I had a 55-gallon tank with various tetras. I had strong currents that were produced by power filters. It was really interesting to watch them line up in front of one of these currents and school in station for hours. It seemed that they really enjoyed the current.

The third reason why water movement is beneficial is because it prevents the formation of a protein film on the surface of the water. You may notice this in your tank, or more obviously, on stagnant ponds or lakes. With time, it can become thick and crusty. This film significantly inhibits gaseous exchange between the water and the air. Are you beginning to understand yet why water movement in your aquarium is so vital?

The next two reasons are perhaps the most important. By having your water constantly moving, you increase the surface area of your tank. The layer in contact with the air is always changing and consequently you dramatically increase the rate at which oxygen can be dissolved into the water, and carbon dioxide is released from the water. The surface area of your tank is always the limiting factor in the number of fishes you can keep in a tank. So, by artificially increasing your tank’s surface area, your tank is then able to house more fish.

The fifth reason why we should pay attention to water movement is because moving water carries oxygen to the denitrifying bacteria in your tank’s substrate. It’s these bacteria that are responsible for the breakdown of harmful waste products (i.e., ammonia and nitrite). Their breaking down ammonia puts it into a form that allows your plants to use it (i.e., nitrate).

And most harmful bacteria are anaerobic. That means that they thrive best in conditions with little to no oxygen. So, by providing a greater air to water interface, you increase the concentration of oxygen in your water, which both your fish and denitrifying bacteria need, and harmful bacteria dislike.

Now let’s talk about how you can begin to create water movement in your tank:

There are countless ways one could create water movement. What I will do is share with you other approaches I've either seen or used that have been effective. In implementing these methods, or in creating your own, it is important to remember why we want current.

Prevent thermal layering

Provide your fish a current in which to swim

Prevent the formation of film on the surface

Increase your water's surface area

Increase the oxygenation of your water

If you keep these goals in mind, you will know what method, or combination of methods will work best for your tank.

The most common approach people take is to add an air stone to their aquarium. This is often done with the belief that those bubbles will somehow cause air to diffuse into the water. Sorry to bust your bubble, but air stones are not going to directly increase the oxygenation of your water. What they will do is disturb the surface of your water, and thus enable more water to interact with the atmosphere.

For large tanks, a simple air stone will not do the trick. For small tanks, like this 5-gallon tank here, it is probably the best approach for creating water movement. For a small tank, you don’t want to have too strong of a current because the water will be like a fire hydrant. Remember, you want to break the surface of your water to prevent the protein film from forming and for increased surface area.

As I said, the use of air stones is the most common approach for creating water movement, but is usually insufficient alone. The next most common approach is to use an undergravel filter. This is supposed to create current by pulling water through the gravel and out through the lift tubes. This method relies upon the power of the rising bubbles in the lift tubes to pull water through the gravel. This alone would hardly constitute an adequate system for water flow. Maybe as a filter it works, but we're not talking about filtration. We're talking about creating water movement.

Undergravel Filter I have seen two adaptations to Under Gravel Filters that seem to work pretty well. One method is to put a power head on the lift tube to more forcibly pull the water through the gravel and up the lift tubes. The water is then redirected back into the tank in a stream. This stream can be made more turbulent by introducing bubbles into the lift tube or power head.

MarineLand Reverse Flow Kit The second method is to use MarineLand's Reverse Flow Kit. This works by pushing the water through the gravel instead of pulling it. I think the second method is best in terms of filtration, but if we keep our goals in mind, we will realize that the first method is the best at creating current. A concentrated stream of water near the top is going to disturb the surface better than a diffuse current rising from the gravel. It will also help circulate the water because it is pulling water directly from the bottom of the tank and spitting it out at the top.

A few people use the Rio Duckbill Rotator. It attaches to most powerheads and slowly rotates, distributing flow 360 degrees. This powerhead attachment is useful because it changes up the current so that it is not always blowing in one fixed direction. It is also very good at overcoming the problem of "dead spots" in your tank. It also meets some of our other criteria for a good water movement system by increasing the surface area of the water and hence gas exchange.

The Sea Swirl by Aquarium Currents is also designed to eliminate "dead spots" in the aquarium. It works by oscillating your return line 90 degrees every 60 seconds. Aquarium Currents has designed them specifically for glass or acrylic tanks, and they come in 3 different sizes (½", ¾", and 1"). They are designed to work with external pumps, although they could be fitted to a submersible pump.These are a few examples of what is offered commercially. What I find to be most fun, however, is to come up with a system of your own. Really be engineering; dream up your own system. I'll show you a few examples of what I have used in the past, or still use.

By having a spray bar, you can create a nice, directional current, and disturb the surface of your water with the return line. You can adjust the strength of the current by the size of the holes drilled in the PVC. The smaller the holes, and the fewer of them, the stronger the current will be. With this particular setup, I had the PVC just below the surface of the water, angled horizontally.

What I prefer to do now is have the PVC above the surface of the water and have it shower the water down, as if it were raining. It is not as effective at creating a significant current but is effective at breaking up the surface. I made the holes in my PVC to be pretty small so that the shower does a good job of breaking up the water. This picture below is of the top portion of my tank, and you can see the shower penetrating a couple of inches into the water.