Yes you will need to drill a return as well.
You can drill it pretty much anywhere you want. Most people put it on the back of the tank to have it hidden out the way. Personally I would do it closer to the edge than to the middle as it is less likely to flex as your drilling. I would put the overflows on one side at the back and the return on the other side.
For teh overflows I would put in two (one slightly higher then the other). This means that if the first one blocks up the tank will fill out to the second one and then over flow. if you only have one overflow and it gets blocked the tank will flood. If you have two overflows at the same level its gives you some extra protection against flooding but then with both of them having water going through them constantly you risk both of them blocking up over time.
IE:
You want a "diamond edged hole saw". Basically it is like a hollow tube connected to a drill bit. The cutting edge has a rough diamond coating which "grinds" rather then cuts through the glass. You will need a variable speed drill (make sure that it isn't in "hammer" mode
) and you just very slowly grind through the glass.
The way I did it was:
Mark where you want the overflows.
Get a piece of wood and drill a hole in it the same size as your holesaw bit.
Lay the tank on a soft, flat surface (a couple of old blankets).
Stick the piece of wood to the tank so that the hole is over where you want the holes drilled (I used duck/gaffer tape to hold the wood in place, you can use clamps if you are careful and pad them).
Dribble a little water into the hole and use it as a guide for your hole saw.
SLOWLY start the drill up. Do not put any pressure on the drill (its own weight should do the trick).
It will make a really horrible noise and gradually start grinding through the glass.
Every so often Flush the hole out with water. I had someone gently dribbling water into the hole while I was drilling as it is important to keep the drill bit cool and the hole free of debris.
Once you have the hole part way started (deep enough so the drill bit doesnt slide all over the place. You can take the wood guide off to make it easier to see what your doing.
Caution:
Be cafeful with water and electric power drills.
Wear eye protection (seriously, the potential for eye damage is fairly high).
Take it slow and dont use too much force. It took me about 20-30 minutes per hole.
/stores.ebay.com/Gloster-Tooling_Dia...1QQftidZ2QQtZkm
