What is being talked about it water turnover. How many times in one hour is the water in the tank being turned over, or moved around?
A 30 gallon tank with 1 pump at 300gph has a turnover of 10 times per hour (300 divided by 30 = 10).
A 30 gallon tank with 2 pumps at 300gph each (600gph total) has a turnover of 20 times per hour (600 divided by 30 = 20).
When considering certain corals you need to be concerned about turnover and flow rates, as some species need more brisk water movement to thrive.
For the most part, fish dont need special flow requirements.
Flow that is too low can cause areas of the tank to become stagnant.
Flow that is too strong can cause corals to become irritated and some fish to have difficulty maneuvering around the tank (mostly seahorses, pipefish, etc).
A turnover rate of 15-25 times is a fairly healthy amount and is probably pretty good for most beginner corals.
Anemones are a different game...they not only require appropriate lighting but also good water quality.
An anemone has the capacity to move to areas of the tank where it gets the flow and light that it seems to like, but you need to provide it with the appropriate needs and then let it "Get comfortable."