I would stop using the Pimafix and just use the water conditioner.
I have stopped about 2-3 days ago. (stinky stuff too :P)
Are you sure they are not tail biters?
I think the white one, Pegasus, that I got might have been. He was the one I thought was there for close to 6 months before I got him. He had nips out of his fins (that looked like nips to me). I don't SEE any of the others trying to grab a their fins though, but who knows. Do you think stress from seeing the other bettas could cause tail biting out of stress or frustration? I don't know much about it, but I can easily block their view from each other if that is the case.
Would also call local water company for info as to whether the water from the tap is treated with chlorine,,or chloramine
The only two additives my county uses in their water is fluoride and chlorine according to the website for our water resources here. they have a chart with everything found in the water and approximately how much they find. Everything else is stuff that gets into the water through plumbing or environmentally.
I might consider placing small sponge filter in tanks holding bettas ,and perform water changes twice a week of fifty percent while leaving the betta in the tank. At the four or five week mark, the sponge filters will have developed some good bacteria to help process ammonia from fish waste (poop), and food and after this bacteria has established itself, then one weekly water change of fifty percent will be all that is needed.
Sponge filters produce next to no current and should not bother the fish too much
this is something I might consider. I have no money right now, but I recently got a job, so I'll see how it goes.
IMO-it sounds like you may be buying fish that may not be in the best of health to start....so you have your work cut-out for you from the get-go....
This is what my BF thinks too. I think it is a major possibility.
Sometimes with hard water, some Bettas fins can be more sensitive than others especially if they came from soft water to start.
I don't advocate use of chemicals to altar water
I myself prefer to use the "the less chemicals the better" approach with my tanks too.