it depends. you'd have to call all the lfs in your area and ask them. some will take fish but no credit or cash (they usually quarantine them), some will give you store credit. varies by store and by fish (one lfs said they wouldn't take most fish, but they might take some of my mollies since these are popular fish that sell quickly, another said they'd take them off my hands but due to being a common and cheap fish, they wouldn't be able to give me much in the way of store credit). and most places won't take them until they are at least an inch or larger. just make some phone calls.