Due to advances in the marine hobby, it is now possible to keep fish and even corals in small tanks like the Fluval Sea Evo and other tanks of similar volume. The factors that affect how many you add include the age of the tank, maturity of the filter, the type and size of the filter, if it has a protein skimmer running and of course the eventual size of the fish themselves. Let’s start with the tank itself.
Many marine fish are territorial and need a little patch of the ocean to call home. That means that a tank that is long and wide is better than one which is tall and thin if you want to mix several marine fish together. Also one with a large built-in filter section or a large hang-on filter will be better than one with a small internal filter which is really better suited to small freshwater fish. So the bigger the filter and the more mechanical, biological and chemical media the better.
Next is how long that tank has been set up for. As good as new filters are they are sterile, so need to be seeded with the right strains of live beneficial bacteria which will then go on to convert fish waste into less harmful substances. No marine fish or invertebrate is tolerant of ammonia so the tank must be ready for their addition and their resultant waste, and that can take a few months unless you fishless cycle.