I understand. Saltwater hobbyists have tanks that are usually either fish only (lower requirements for success) or living reefs full of coral. If you have ever been on a real reef, you will find the water full of small particulates, much of this matter is usable food for filter feeders like coral. In a fish only system, these particulates can be filtered out without harm as the fish could care less. A fine filter is used in the system to capture these tiny floaters.