Your brita filter isn't replacing a conditioner. It only removes some things, and not all of anything. I would stop using the brita filtered water at all and just use tap water. You also have to invest in a conditioner which says it removes chlorine, chloramines, and heavy metals. This is a big part if not all your problem.
A baby betta needs better quality foods. I would look into something like NLS Growth formula. I am concerned he/she is being stunted by your feeding habits. You've had him 3 months and he was old enough to sell at that point so he should be looking more mature at this point.. Hikari are low quality wheat based pellets that in my experience cause constipation issues..and that's with healthy adults. Blood worms are okay treats once a week (in small amounts) but anything freeze dried needs a thorough soaking for 10 minutes to soften before feeding. Your betta should be fed daily in 2 small meals split up and one fast day a week.
If you change your feeding habits and get your water quality under control he/she should be a lot healthier. I also don't know about keeping one in something so small without a filter because I do know that even with this water change routine he/she will be getting traces of ammonia. You may need extra water changes but someone will need to comment on this as I don't have much experience.
Last edited by callistra; 01-15-2013 at 10:52 AM.