Hi Wendy-
Welcome to the group - it is really nice to meet a fellow Betta owner. I have used Bettafix and I thought it did the job. It is definitely made for the SMALL tanks as it would be too expensive for any of the bigger tanks, but it is easy to use and didn't cause further problems. I keep it for my quarantine tank for things like fin damage and such where he needs to come out of the big tank for a while.
My biggest problem has been with the plastic plants because he is so CURIOUS and wants to squeeze in between everything, so I am in the process of switching to real plants completely. Hopefully, they will be more gentle. (I have him in a 5 gallon tank now)
Gunnie and some of the others can advise you more about the bigger tanks as I don't have any experience with anything bigger. (They also have have a LOT more experience. My little guys would be in fishie heaven by now if it weren't for them.)
Again, welcome.
Shalom,
Rose
