How often do you can the filter and water in a 3 gallon tank with a 1-3 gallon filter?

I have a tank that is 3 gallons with glow fish in it. I cycled the tank and the fish are doing well. The water is getting a bit mercy and I don't have an allergy eater. It died because I don't think their was enough for it to eat in the tank when I put it in. The tank has an under gravel filter and a standard filter I put in it. How often do I need to change the filter cartridge and water? Also when I change the water do I have to let the tank cycle again before re-adding fish or can I just put in the old filter sense it has the bacteria the water needs? Also when can i add another allege eater?