I feel a little better now. He had been living in about 250ml of water for at least a month, and what ever size they had him in at the breeders before that... When I bought him he went straight into his tank... The tank is 17L... Which according to google, is just under 5gal. It just has a little corner bubble filter. I had one sitting around from when I originally bought the tank 2nd and hand figured it would create much less current than the filter I originally bought for it.
I think it is all the zipping in and out of the plants what is doing the damage. He loves to swim all over his tank. He totally freaked out when I put him back in his cup to do a water change, I've never seen a fish freak so much in my life. Hopefully his tail will grow back stronger and will stop breaking.
Now I just need to I need to find something to put in there for him to play in and around and give him some cover until the moss gets bigger.