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Water Changes in the Cycling Aquarium

Don't Water Changes Keep the Biological Filter from Maturing?

(The First Tank Guide)

Some people recommend not doing water changes while the tank is cycling because they believe that this will disturb the bacteria that are colonizing the biological filter and will either prevent the tank from cycling or will make it take longer to cycle. This is actually one of the common misconceptions in the aquarium hobby.

What Are the Benefits of Doing Water Changes in a Cycling Aquarium?

Not only does performing frequent small water changes in a cycling aquarium not delay or hamper the cycling process, but, in many cases it seems to speed it up slightly!

In addition, if your tank is cycling and has fish in it, the elevated ammonia and nitrite levels are almost always fatal to the some, if not all, of the fish before the biological filter is up and running if you do not perform regular, frequent, small water changes to keep these toxin levels low.

Performing small, frequent water changes while the tank is cycling is not only beneficial to the cycling process and helpful to the bacteria you are trying to establish, but it is also crucial to the health of the fish in the tank.

What if I Don't Do Water Changes While My Fish Tank Is Cycling?

Failing to perform these water changes while the tank is cycling is inhumane and cruel, and highly irresponsible. Anyone who is questioning whether you should be doing these water changes while the fish tank is cycling, or who is telling you not to do water changes while the tank is cycling should be questioned.

How Do Water Changes in a Cycling Aquarium Speed Up the Cycling Process?

Elevated ammonia and nitrite levels in the water are not only directly toxic to your fish, but also displace oxygen from the water. When ammonia and nitrite levels get high enough, there may not be enough oxygen dissolved in the water for the fish to breath.

Along the same lines, the nitrifying bacteria that are trying to populate your biological filter require oxygen in addition to ammonia (or nitrite) and water to survive. By allowing the ammonia levels to get too high, not only do you suffocate your fish while simultaneously poisoning them, but you also suffocate the biological filter, making it harder for the bacteria you are trying to grow to thrive.

Why Do Some People Say Not to Do These Necessary Water Changes?

The most common explanation that people who recommend against water changes in a fish tank while it is cycling is that these water changes will disturb the bacteria, and either prevent them from getting established in the tank, or remove them from the tank so they can't get established.

However, the bacteria in question are pretty good at holding on to surfaces. They usually are holding on to surfaces in the biological filter media which sits in flowing water being moved by the pump for the filter. In most cases, this water is moving faster through the filter than it will be removed from the tank by your siphon. (Remember, you want to turn over the entire tank volume 3-5 times an hour in most filters. In order to approach this current, you would need to empty the entire tank in under fifteen minutes with your siphon!)

So, you won't be removing the bacteria from the tank in any meaningful quantity with your water changes. but that can't be the only argument against doing water changes and saving your fish, can it?

No. They also argue that by lowering the ammonia levels in the water that you will kill off the bacteria. Now, if you were removing all the ammonia from the water and preventing any additional ammonia from being released into the water this would be a valid argument. However, when you remove 10-15% of the water, you're only removing 10-15% of the ammonia from the water. The majority of the ammonia is left in the water, leaving plenty for the bacteria to consume.

You also leave the fish in the water, so there is additional ammonia released into the water immediately. Though the biological filter will die off in fairly short order if it is starved for oxygen or waste, it will not be starved for either if you keep up with the extra water changes necessary while your fish tank is cycling. Of course, as mentioned above, filing to do these necessary water changes while the aquarium is cycling will deplete the oxygen levels in the water, and can actually result in hampering the growth of the nitrifying bacteria.

People also argue that the elevated ammonia and nitrite levels in the water will provide more food for the nitrifying bacteria. While this is true, with the decreased oxygen levels in the water due to the increased ammonia and nitrite, the bacteria cannot process the waste to grow. The elevated waste levels in the water slow down the cycling process.

Remember it is very important for the health of your fish and the health of your biological filter to perform regular frequent water changes while cycling your aquarium.

"Thank you so much for such a quick response!! I took off the impeller [from the power filter] and cleaned it good. I didn't even know about that. I thought I was going to have to replace the whole pump. But it is working great and the tank is so much clearer. Clyde seems happier too! Thanks so much for your help."
April 7, 2002
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"Cool page you have Keith."
March 19, 2003
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