My fish has the same problem! I've actually posted about it twice before and never got an answer as to what to do so I am so happy that I found this thread! (I'm posting so I can follow it easier ) I'm going to also give it a try. I currently have my fish in a 5 gallon tank and I thought his filter may be too strong. I baffled the out-flow side but the intake side was never touched. He hangs out near the intake though, could that have caused the similar problem? I just bought a 10 gallon tank for him and I'm worried that the new tank will have even worse of a flow so I'll have to figure something else out for him.
Edit: I just read part of the thread that MSG linked to. It answered my question about the intake tube. (Answer was that the intake tube can cause damage too.)
The easiest way to make sure the intake tube is safe is to wrap it in nylon. I do this for all my betta tanks regardless of the filter brand, and I've even done it with the whisper in tank filters- just wrap the entire bottom. For a regular intake tube you could also cut a hole in a piece of sponge and stick that on instead; plus this gives the added benefit of increasing the surface area your beneficial bacteria can colonize.
If all else fails and your filter is still too powerful, you could always replace it with a sponge filter as these are probably the safest for long fins.
Hi, I am not sure if you have tried this, but I have used some Amazon Sword plants to keep my Crimson out of the current, and so he won't get caught in the intake, and It has worked, I noticed his fins started to frey..and I knew It was the filter, I have a 5 gallon as well, came with a "Mighty Might" filter..and I beleive It only shoots water out, but because bettas love being curious they go to the filter intake, but AQ salt and warm clean water will help to regrow the fins, you could even do the warm water without salt for the first 3 days, salt may not be necessary..but do not use the "FIX" medicines..they will cause problems..
also just to add, I have the Pinapple decoration, and the Flowerball in 3 of our fishy's tanks..