Internal vs. External return pump...

[drparker]

Also, there are two holes on the bottom of the tank, hidden by the overflow. It was suggested earlier to use both as drains and PVC pipe the return up the back of the tank. Won't the two drains overpower the draining capacity of the slits in the plastic overflow, if I did this?

No they won't. You would use the stand pipes, durso style, to control water height and noise in the overflow. The slits should be able to handle up to about 600 gph, anything under that and they match the rate of the return pump.

Like already stated no reason to use both as drains unless you want 600pgh or more of flow and then you'd need to make more slits or make them bigger. I only use one for the drain and the other for the return with an Ehiem 1260 on my 75g.