From my limited knowledge about RAS systems, you have to size the filter according to how many pounds of fish or rather how many pounds of food you are feeding per day. Not how many gallons of water are in the system.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).