Ooh, so the salt crystals weren't aquarium salt but for water conditioning? Aquarium salt here in the States is for medical purposes.
Ah, the fish appears to be a tiger barb.
Yeah, I really think a lot of the problem came from changing the filter pads. Some people do change them regularly but others just rinse them out weekly and that's it. If you do change them, it helps to change only half at a time. That way you never llose all your good bacteria.
When you do get another betta, you may want to put him in his own heated, small tank, maybe around 25 liters or so. I hope you are able to get another one, they are so much fun. :)