it is pretty much overfiltration for a 20 gallon tank.. the Emperor 280 can do anything from a 35 to a 55 gallon tank..
a 20 gallon that is a heavily stocked fish-only tank would need about a 200gph filter..
a normaly stocked fish-only 20 gallon tank would need about a 150gph filter..
a 20 gallon planted tank would need a 100gph filter..