Closed Loop Circulation

If the hole is on the back wall and lower than your tank's water level (and the pump is lower than the hole), then there will always be water in the line going from the hole to the pump: So, your pump will always be primed.


If you don't want to drill the back of the tank, you could use a "U" shaped pipe to sit over the top of the tank (sort of like a hang-on skimmer/prefilter for a w/d). BUT, you have to make certain that the air is removed and water is filled inside the pipe to the pump. And, when you do water changes and the water line goes below the opening of the "U" inside the tank, you'll lose siphon and you'll have to reprime. (But, that's not too big of a deal.)

I have a sort of closed loop with a "U" pvc pipe drawing water from the tank and sending it to the pump. But, I have multiple lines feeding the pump: 3 total and the other 2 are always primed.
SO: I don't prime the U pipe after a water change. For the first couple of seconds, the pump draws in air until that line is primed. BUT: the pump is not running dry during that time since it's getting water from the other 2 sources.

So, I wouldn't recommend this to you if your set-up will be just one source of water to the pump.

Also, bear in mind that most pumps are designed to have minimal pressure on the intake (no restrictions). So, don't have too far of a line going from the hole you drill to the pump. You can have a lot of line (depending on your model) going from the pump back to the tank. The better pumps handle back pressure very well.

HTH.
Richard.