I would hassel with it "now" than not doing it and have wished that I did do it. Is the tank glass? Acrylic?
Glass- You can use those diamond burrs that Harbor frieght sells for 5-6$ for 20pcs to make the hole bigger.
Acrylic- You can use the same thing, or use a Fostner type bit (woodworkes) to make it larger. A sanding drum works well here but is a little slow.
Just make sure you have the Bh "in hand" before you make the hole larger. You could just add another Bh to increase the water going out, if you have the room.
Or live with what you have and add a CL for flow. That is where the "flow" or "turn over rate" would come from. Why don't you want to drill the back of the tank for a CL

eaiser in the long run. Checkout this post, his design is taken from several others and incorporated into this:
http://www.reefcentral.com/forums/sh...hreadid=371622
good luck