I got a male crowntail from Petco and put him in a new 2 1/2 gallon tank with this filter
http://www.aquahobby.com/products/e_whispermicro.php He looks like he is having trouble swimming, and I am not sure if he is just ajusting after being in that tiny cup or if i should pernamently remove the filter. I am afraid that if i leave it in it could cause too much stress for him
If you can turn the filter down do that, if not put objects like plants in the way to diffuse the current somewhat. It often takes a betta a few days to adjust to any current after living in a petstore cup for so long.
I agree. Eventually they bulk up, and can handle the current, if it is slowly increased, but I think they atrophy, because after they are collected and jarred, they never really have to fight a current.