Water Changes (Cont.)
Large fish like this oscar (Astronotus ocellatus) can produce a lot of waste that will eventually add up to a massive amount of nitrate in a short time. Weekly 75% or greater water changes may be requaired in a aquarium ecosystem with these large fish.
In cases where you might be keeping very large fish, like oscars (Astronotus ocellatus) where nitrate tends to accumulate very quickly, lower the tank water to the point where the fish just has enough water to keep wet, then fill the tank up to say 50% full, then drain again. Fill water at one end of the tank, and drain water out of the other end. This will help send nitrate rich water to the end that is draining the water. You can repeat the steps several times to try and get the nitrates as close to 0 ppm as possible.
How often you will need to do a water change on a aquarium will be directly related to how fast nitrate accumulates in the system. On an established aquarium, where standard fluorescent lights are used, and does not have live plants, weekly water changes may be required to keep nitrate levels below 100 ppm.
In aquariums that have high output lighting systems with fast growing live plants, and low fish population, it is possible that the nitrate will never accumulate. In this case you may only need to do a partial water change every three to six months and/or adjust the chemistry between water changes.
You should test your aquarium water after a water change for nitrate, then again a week after to see how fast nitrate accumulates. This will give you an idea of how often you will need to change the water in the aquarium.