That's something I'll be learning as well about the PFS, I know a guy that has a multitude of tanks & he's running PFS on all of them. I have to stop by when he does the maintenance so I can check it out. He highly recommends the stuff.
What kind of food are you feeding your cories?
Also my Rosy barbs did NOT like the bettas at all. Cherrie B's may be more docile, but that's just a guess because they have whiskers & are probably more bottom feeders.
Get a 50lb bag of the PFS, but shop around first. I heard some brands require more cleaning than others.
Definitely don't buy a fish if they look sick. Treating for disease in a new fish is too much trouble.
My female betta was transfered to my guppy tank last night to give her some exercise. She ended up decapitating the smallest guppy this morning, but the rest of the guppies are unharmed. I think she ate one of the snails too.
*sigh* Time to transfer her.