I don't use filters. I did, but I noticed me bettas seemed to not like them. I had one tail-biter who still chomped his tail after I took the filter out but not nearly as much as he did when it was in his tank and running. I use API stress coat as water conditioner. :/ I was thinking about switching to prime, but the store near me never has it in stock, so I've been sticking with stress coat.
I wanted java moss, but they didn't have any at either of the stores I went to. I should've known better, but I listened to the employee's advice on the water wisteria. She told me it was popular because it was easy to grow. I'm a little skeptical that it was already dying in the store, though, because I only put the stupid thing in two days ago. I'm thinking of going back today. I'm either going to get more moneywort, because that seems to be doing okay, or I'll see if they have hornwort or java moss. I've heard hornwort is pretty tough to kill, too.
I got these three pretty recently. I've only had Mozart and Brahms for about two weeks. But it's been pretty consistent that my fish have been having problems. They've all come from either Petco or Petsmart, but some have come from different locations. The fish I had while I was at school came from the ones near Princeton. Now my fish are from the stores near home.
But one of the fish I know of that seems to be doing fine in a .5 gallon, unheated tank is a betta my aunt found at a dollar store. (Mind, the dollar price included the fish and the stupid little container.) So I'm sure that the care that fish got in the store was even worse than most pet store bettas.
Maybe it is just something about the clean water upsetting the chemistry...
But sometimes it's just been really sudden things. Lucius, my most recent fish to kick the bucket, was fine for months. Then all of a sudden one day he was clamped, sulking on the bottom of the tank, and ignoring food. That night, I got him to eat and he wasn't as clamped so I thought he was going to be fine. The next morning he was clamped and sulking again and later that day he was dead. I have no clue what happened! *sigh* Maybe eventually I'll figure it out and start doing things right.