5 is a good number, but 8-10 is better. 20 is better again! 
6' length minimum, width isn't as important really, 18" wide would be ok, but 2' would be better. Try set the filters up to create a good current in the tank too.

6' length minimum, width isn't as important really, 18" wide would be ok, but 2' would be better. Try set the filters up to create a good current in the tank too.