Grab a little chunk of styrofoam and use your hole saw to cut through it. Then clamp it onto your tank as a template, it'll keep your bit from skipping around. I've done the angle method as well and the template makes it a lot easier.

Thats a great tip Sean....

If you want, have a look at the first couple pages of my tank journal http://www.canreef.com/vbulletin/showthread.php?t=40669 ....theres some fairly detailed pics of my herbie set-up.

also take a look at Dez's build, some good pictures there of working with glass.

Hi sean thanks for the tip. Im thinking im going to get my other half to do the cutting so am making a list of tips for him. I think he will do a few practic runs and will deffinitly get him to try that. It sounds like thats a tricky part. Getting the bit to stay at first. And Fishytime, great tank. Im only half way through your thread and it looks amazing!

Another tip is to set the clutch on your drill to a very low setting so if the bit binds up at all the clutch will kick in and hopefully prevent it from cracking. Drilling holes is easy, just take your time and keep the bit wet. Also don't drill to close to the edge, whatever size you are drilling leave at least that much space between the hole and the edge of the glass.

drilled a 20g and 33g with a Dremel for 1" bulkheads, took about 15-20min per hole for going slow, packet of diamond bits was ~$10, some to consider also

Just a couple of additional thoughts regarding this diagram...

1) The primary drain line (the one with the ball/gate valve) is shown in this diagram to have a tall standpipe up inside the overflow box. That pipe isn't necessary. In fact, I don't think it is a good idea. Optimally, the water level in the overflow box operates at some point which is ABOVE the primary drain, yet BELOW the emergency drain. In this diagram these two drains are very close in height, providing little room to operate.

Further, if there isn't enough water depth directly above the primary drain, then it tends to form a vortex and suck air down...which defeats the entire purpose of the design.

2) As I just mentioned, the diagram shows the water level at the level of the emergency overflow. That's OK, but the system works more quietly at a water level slightly below that.

If you ever want to come by and look at a variety of Herbie overflows in operation, just let me know. In my refugium, I have a standpipe on the primary overflow. It is there for a very specific reason which I can demonstrate to you.

There is a advantage to having a short standpipe on the primary in that though the Herbie method is silent, you can still get some noise from the water dropping into the overflow chamber.

This noise can be limited by lessening the drop (by raising the water level in the overflow chamber).

Another small tip regarding drilling the tank. Masking tape is a good idea, but for extra insurance place something soft below the area you're drilling just in case the tape doesn't hold. Even a piece of cardboard will do, but something a little thicker will be better to catch the 'donut' hole.

Hi, Thanks for all the great advice, im just trying to track down a local drill bit (trying to get around waiting for the shipping) Seems easy to find 1 1/2 inch bits and 2" bits but can not find a 1 3/4 inch that i need for a one inch bulkhead. When i track one down i will be sure to use all your advice. Thanks for all the replys! Do you think i could drill with a 1 1/2 inch bit and use a diamond coated dremel to make it fit a 1" bulkhead? Or is that to risky