Have you checked the temperature of the water? I came home from vacation once and found my tank heater stopped working. Fish were cold and sluggish. Once I got a new heater and warmed them up they started swimming normally again. If temps are okay then you may want to run a test strip test to check your nitrite levels, amonia levels, pH levels. Good websight to get more answers from is www.aquariumlife.net and www.wetwebmedia.com Best of luck!
Go with answer #1; don't listen to #2 since fish who are oxygen deprived do not stay at the bottom of the tank but rather will swim at the top literally gasping for breath.
It may be because there are not getting enough food or they are sleeping. You can't really tell when your fish is sleeping because they sleep with there eyes open. It also mean that the water can be too acidic. Be sure to check temperatures and clean the tank at least twice every two weeks
Your fish don't have enough oxygen in the water you will need either a pump and air stone or a stronger filter if you have one.You will need to clean your filter or air stone at least once a month in fresh water.