He's probably suffering from high ammonia and/or nitrite. Waiting for your snail to act sick is a BAD way to determine water change schedule.
Most people will tell you that you can't ever fully cycle a tank this small, despite your filter, and you will always need these twice weekly changes of 50% and 100%.. you can try, but careful daily monitoring and reliable test kits should be used. You need to be testing daily with a reliable drops kit for ammonia and nitrite and doing an extra 50% change any time you see either. In addition to this a weekly 50% with siphon or new fresh turkey baster that has never seen chems is needed to remove poop and other debris from the gravel. It is not enough to just scoop water off the top ever. I actually suggest a turkey baster or very small siphon because your tank is so small an average siphon will remove water too quickly.
First you will see ammonia, then nitrite. Eventually, hopefully, you will see ammonia fall and stay at 0 even after a week of no water changes, and finally nitrite. At this point you will be left with only nitrates after a full week of no changes and these can be kept <20ppm by twice weekly 50% change with baster/siphon. However, cycling will take up to two months to complete and many if not most people will tell you that you can't cycle a tank of this size and you will always need twice weekly 50% and 100% water changes or you will always see ammonia continuing to build and the cycle will never finish..
At this point I would keep up the regular 50% and 100% weekly changes until you can test.
This talks all about the problems with smaller filters and replacing all the filter media at the same time: http://www.bettafish.com/showthread....on#post1424758