Do you buy used fish tanks I have a 90 gallon tank I'd like to sell.. in the tank there's 2 catfish and one giant plucostamus.

You should call ahead of time to make sure they have tank space for the fish. Not all store will take fish on just walk ins

Talk to them about the tank. You'll want to take the fish in first, as long as they're in good health you can just take them in to the shop in a good amount of water and they will make you an offer on the spot. That's been my experience.

Just call they are friendly and accommodating

They do but used fish tanks. You simply need to call and set up an appointment with them. They also take fish, however you need to speak to them about either buying them or donating them.