Welcome to Gaia! ::

Betta have a life span of about 3-5 years so that's actually very good! I only kept mine alive for a few months ;;

The waste won't really float too well to the filter. I thought the same thing and then when I finally removed my gravel... I was surprised. I prefer large smooth stones instead of gravel.

You don't necessarily need gravel. Right now my 20g is bare bottom because it was a pain.

Sometimes some fish just aren't healthy to begin with though. Depending on where you got him he may have been inbred or overbred or any number of things. It's especially hard to know where your fish are coming from in generic pet stores or even grocery stores that keep betta in those little plastic cups.

It might not be anything you're really doing wrong but frequent water changes in any size tank is a must. Sometimes in bigger tanks that might be understocked it's not necessary to do huge water changes once a week but the idea is that you don't want your fish sitting in too much ammonia.

Do you rinse your filter media in the siphoned off water? I tend to keep my filter media for as long as I can since there's so much bacteria in there.

Stay positive! You can't help that your fish may just be unhealthy.

I've only bought one fish from petsmart and he seems okay but it all depends on how long they're in those little cups.

If you can find a higher end local fish store you may end up spending a bit more on the fish but if it's healthier, that's best.