A couple of you people posting that you don't do water changes, or do them only once a month, are heading for a severe case of OTS down the road.
OTS = dead fish and you won't even see it coming.
When you try to introduce new fish, they will just die. Bang. Boom. Dead. You won't have a clue why. Unless you test your water for nitrates, but most of you won't.
One day your older fish will "look" fine and swimming around. Next day -- BOOM!-- all your bottom feeders will start dying. Then the rest of your fish. Then, unless you get your butt on here and start asking questions, your tank will be empty and you'll start trying to add more fish. And they will die immediately.
Lemme tell you what happens when you don't change your water out regularily -- and that means WEEKLY -- not every two weeks, three weeks, or when you get around to it -- WEEKLY.
Course we all know that bacteria eat ammonia and excrete nitrite. Another buncha bacteria eats nitrites and excretes nitrate. So what kind of bacteria eats nitrate?
There isn't one. So, you have nitrate in your tank that can only be removed by water changes. But, that's not all of it. You also have TDS -- Total Dissolved Solids -- and DOCs -- Dissolved Organic Compounds -- which we have no way of testing for in the hobby. Both TDS and DOCs are a natural part of the aquariums processes. Again, those can only be removed via water changes.
Unfortunately, people use nitrate tests to tell them it's time to change the water. The problem with that is that affordable nitrate tests in the hobby aren't all that accurate and they don't measure DOCs and TDS.
If you do not change your water regularily, your fish are pretty much going to swim in liquid jello. The less changes you do, the more like setting jello the water will become. This will affect the fishes' ability to osmoregulate -- adapt to changing physical conditions in the aquarium. So the fish in the tank adapt to the water that's in the tank, but if anything changes or the water goes beyond what they can tolerate, they die. Boom.
More on OTS:
http://www.thepufferforum.com/articles/small/ots.html
You need to change your water WEEKLY in amounts of 25% or more. Not doing so jeopardizes the health of your fish.
Roan