Do your homework and decide if they are right for you, to my knowledge, most are short-lived in aquariums. I haven't put a lot of time in keeping clams (well, in freshwater) but I can attest that they FOUL up a tank when they die. And as filter feeders, most present some hurdles to feeding and maintaining water quality. I'm not sure that there are many great candidates for a 5 gallon. Put them in a spot of good current and keep an eye out daily lest any perish so they can be immediately removed.