Okay that actually points to your problem, and it's a pretty simple fix.
Aquariums and fish tanks are closed systems, which means that what goes into the water stays in the water. All of your fishs' poop, u-eaten food, and other waste accumulates in the water.
Water Parameters are the numbers relating to the basic levels of Ammonia, Nitrites, and Nitrates in your tank water. The higher the levels, the more unhealthy the water is for the fish. I'm not sure what time it is where you are, but getting your water checked, or doing it yourself with a water testing kit would be a very good idea. Petsmart will do it for free if you bring a sample of water for them to test.
When you just top off the tank when it is low, you don't get rid of any of the toxins in the water, which build up and will eventually kill your fish.
Do you have a water conditioner that you use when adding water to your tank? If you don't, you need to get one. The only way to help your fish is to take some water out and add fresh, conditioned, temperature matched water. What size is the tank?