Is 540 gph good for a 20 gallonHow many gallons is your tank? You need a 6-10 times turnover ratio minimum. Some even like higher, especially if you run/want to run SPS. So for example if your tank is 10 gallons you want between 60 and 100gph per hour.
EDIT: For another example for "higher" turnover my 125 gallon tank has around 3000gph+ going through it. Which is a 24 times turnover. Some go a lot higher then that. Flow is good, the only time it's bad is if your fish are basically sticking to the walls LOL