Yeap, what PedroYoung said. Also, in my case, my pulse mode give me over 5" water wave and splashing water out of the tank if I run it at 100% in my 6' tank. On the other hand, I need close to 100% in some other modes to get the entire tank water moving. So I program mine accordingly.
Do you have the 40's or 60's on a 6ft tank? Also, are mounted on the sides or back glass? And how many?
Just trying to get an idea for placement when my tank is good to go.

