plugging bottom drains?

[JO JO]

The 135g I purchased was setup as SW and has 3 holes drilled in the bottom of the tank along the backwall, out side of the overflow. i am think of capping off all except one to use as a return, but i'm skeptical about that one if a power outage was to occur. The is my first experience with a drilled tank, so not sure what to expect.

any suggestions on my situation?

 

[ercnan]

As long as you use some sort of check valve/back siphon device, or even two in case one fails,you should be good.
You could one of the others for a tank drain, and the last one for a closed loop circ. system.

 

[fishfoot]

To plug them off, you can use 2 methods. On is to get bulk heads with valves so they can be opened and closed. the other is to seal them off with glass and silicone. Get good 1/2 glass or even 3/8s to be sure and then get some GE 1200 silicone. Make sure the glass that you will cover the holes with has a good 1/2 clearance over all sides of the hole and then silicone the glass over the holes so you have created a patch. this iis pretty much an easy thing to do and can be reversible if you want to do it ( get a good knife and cut out silicone from patch0; but it is more secure then the bulkhead/valve system.
As to power outage are you connecting the over flow to an open sump or a closed system I am not sure from your post. Let m know.
good luck