I put together a betta sorority about 6 months ago and purchased 7 female bettas and 4 cory cats. Everything has been going great and 6 of the 7 females have grown to what I would consider full size (double what they were when I purchased them from petco) Even the smallest runt grew to be a decent size. The 7th and final female is the issue.
She will not eat 2 of the betta foods I supply (Attisons & NLS pellets) but eats bloodworms and sinking pellets I feed the cory cats. Behavior wise the fish seems lively but doesn't react like the rest when its feeding time. Normally the 6 females will come all the way to the top and beg/wait for food but this one comes mid way up at most but would rather wait at the bottom with the cory cats. I'm concerned the fish is sick or something along those lines but I'm not versed enough to be able to determine that and was hoping to get some input here.
If there is any other details that are crucial to find out whats going on just ask and I'll try to fill in any voids. Basics below
I do 1 week water changes (gravel vac) and keep the temp between 78-81 degrees. I treat the water with Seachem Prime once every 3 days. I test the water weekly prior to the water change to ensure everything looks good. The bettas are not aggressive at all when they were originally placed in the sorority the normal fin nipping and things of that nature occurred but its been months since I've seen any bullying or fin damage. The tank is heavily planted and is 20 gallons in size.
Thanks in advance!