He's getting old. :( All you can do is make him comfy and warm so that he can die a happy old man.

What is the water temperature? Water under 72F can make him lathargic.

Hi!
I stopped the filter for a few days and he is no longer only at the bottom of the tank. The water temperature is 70 degrees, so I will try and warm it up for him. Pippen says Thanks! for all the help!

Since they're tropical fish, temperatures of seventy degrees for water temperatures are a bit cold for them and most tropical fish will tend to get lethargic, when water temperatures are in the low senventies even, and 78-80 is about minimum for most tropical fish.

If your aquarium is cycled turning off the filter may cause the aerobic bacteria to die. This could be a problem if you have gravel, the anaerobic bacteria takes over and will cause tons off problems. Try placing something to difuse the return flow, like a floating plant, plastic or live.