Silverado61 is rt on. I run 4 cartridges in mine and usually every month month and a half change them out in rotation. I may also be crazy but I get alot of pods in my filter so before I toss the old one I let it sit on top of my filter to drip dry and allow any pods to hop off and into the filter/tank.
over stocking is a tough question. A hippo tang would over stock a 90, but a few fire fish, pair of clowns, fox face, yellow tang, wrasse, mandarin, scooter blennys, ect would be fine. I just listed my 75 stock btw lol