I have a March DP-6T-MD pump. upto 38 gpm of flow. used for 1 year on a SW tank as a closed loop. It is not quiet. A bit louder than a Mag 36. Moves the water nice tho. Would be good for a larger tank.
Flow rates range from 0 to 38 GPM maximum. Feet of head range from 0' to 46 maximum.
http://www.marchpump.com/six.htm
Looking for around $130 for it. Will ship
In Vancouver B.C.