soundslike you love your fishies. you're doing alot of right things, however (as we've all learned) bettas don't really like change. too many new or different things within a short period of time will stress them out and make them susceptible to things like Ich (or Itch), fin-rot, fungus, and other gross stuff.
your water changes sound good, i would start putting some aquarium salt (NOT table salt or ionized salt), at the recommended doses. that helps keep yucky diseases like Ich away.
Your fishie friend soundslike it may have a bit of fin-rot. catching it early is good! try something like MelaFix or BettaFix. follow the directions.
also, you might want to make sure that you are using a dechlorinator that not only gets rid of chlorine but also chloramine. if you're not sure if your water has any chloramine you can either call your local water department or just get some stuff that'll rid of both.
oh oh and let's not forget feeding. bettas need a varied diet, so giving them just one type of food all the time isn't really healthy. think about if you would eat one thing all the time (that's what i do, yuk). yeah so... blood worms, black worms (less messy and use a medicine dropper or plactic pippette), vitamin flakes, Hikari Bio-Gold pellets (everyone's favourite), are usually good things to feed a betta. most importantly... don't over feed your bettas, no matter how much they beg! it pollutes the water and isn't good for bettas, like too many fatty cheesburgers for little girls.
oh oh before i forget, if you have gravel, "vacuum" or suck up with turkey baster or siphon method, to get all the uneaten food and poops out of gravel.
and there you go, on your way to happy healthy betta. you're other best peice of advice is to go through all the articles and fish forum pages. most people have all thier questions answered by reading everything.
good luck and let us know how he progresses!
ENJOY!! <img src="/images/graemlins/laugh.gif" alt="" />