What are these drilled holes for?

Yes that is for a closed loop. Now all you need is a pump & some fittings & away you go!

 

Looks like they used the bottom holes as intakes and the top as outputs on the closed loop.

Thats a cool tank, how big is it?

 

Well it depends on how big your return pump is. The water should go up about 1/4 inch or so on the teeth. The holes in the top usually would have that loc line (the black platic tubes you allready have.
Then you can aim the nozzles wherever you want. This will help flow & you can zero in on dead spots.

The pump can go underneath the tank. The return line can go up the back of the tank & T off from there.

If you dont like it, you can plug the bottom holes & just use the regular over flow & power heads for your flow.

 

They probably had loc-line hooked to them like the plumbing in the pic. Your probably going to have the water level at least an inch below the top.

The plumbing would run behind the tank and over the top, I would hook it up as your only other option for flow is to use powerheads. The closed loop would look way cleaner and there would only be one pump to maintain rather than a bunch of power heads. 

 

see keep that pump your trying to sell and use it for a closed loop here

 

the top holes might be to run cords for power heads through. I driiled holes in the top of my tank so the cords can go straight down to the power heads

 

Was that the same tank on Ebay? I belive it is, I was looking at it for a while, but ended up getting AJ's 90 instead.