If your tank is cycled, meaning it has a population of bacteria that keeps the ammonia levels in check, you will need to do frequent, smaller water changes, about 10 to 15 percent every few days. Changing all of the water at once will destroy the colony of bacteria that you worked so hard to build. If the tank is uncycled, more frequent, larger water changes are necessary because you will need to remove all of the ammonia buildup manually. Where does the ammonia come from? It comes from fish waste, decaying plants, and leftover food. You can test for ammonia and nitrate levels with a testing kit from your local pet store, or you can bring them a sample of water and ask them to test it for you.

On the Other: Size and Number of Fish

The frequency with which you clean your fish tank also depends on the size of the tank and the number of fish kept within. Each fish you add to the aquarium will produce waste that goes directly into the water. If the tank is 10 gallons in size and only contains one betta fish, you may only need to change 25% of the water once a week, with larger changes every month or two. If, however, you have a 3 gallon tank that contains five small fish, the greater waste production in a smaller space will necessitate more frequent water changes. A tank siphon will make larger water changes quicker. Remember to move the fish to a safe place before doing a 100 percent water change.

Bottom Line

If your fish are healthy and the water is clear and free of odor, your maintenance plan is working. Keep an eye out for cloudy water, sluggish or gasping fish, fin damage, or a strange smell. Any of these things will tell you that you need to clean a larger percentage of water more frequently. You should also pay attention to the filter and change the media regularly, particularly if you notice a slower water flow.

Source:

Aquarium Fish Tanks

Cleaning Your Tank

Cycling Your Tank

Resource:

Cleaning Your Tank