Compatibility is one of the most important parts of keeping a marine tank successful. If you have one fish bully the other weaker ones they will gain stress having more of a chance to get a disease, and will then spread to the rest of your livestock. Before buying fish, you may want to decide of the kind of fish you want so you don't end up buying a really aggressive fish and then later on you will want community fish. If you have an invertebrate tank you may want to get small fish that aren't territorial. Big fish may damage or eat some of your corals. The amount of fish you can get is about 1 inch of fish per 2 gallons of water in a fish only tank. For a reef aquarium the rule is 1 inch of fish per 6 gallons. You do NOT want to overstock your aquarium. That will cause to much bio-load and will kill all of your fish. You also may not want to get many fish for a reef tank because some can get a disease which requires copper based medications. This medication is deadly to corals and inverts.