Where to Drill Overflow Bulkhead

inch and a half is a HUGE hole. I have a 1 inch on a 40 that cycles 700 gph with plenty of overhead to spare. With that being said what kinda tank is this going into?

With that being said some drilling basics are as follows:

Avoid drilling in the bottom of the tank, it is often tempered glass. Additionally, the water weight on the bottom is significantly higher than on the sides of a tank
 
Micro fractures always occur anytime glass is cut or drilled, and the hole reduces the structural and tensile strength of the glass.  While aquariums are generally engineered to be much stronger than actually needed for their capacity, the bottom is the "weakest link" and is best avoided.

Always Drill at least 1" from the edges or corners of the glass:  All glass contains minor imperfections and flaws and again, micro fractures occur along the edge of glass when it is cut.  To avoid placing any additional stress on these weak spots, try staying at least 1" away from any glass edge.

and remember that the force of the water against the glass is greater at the bottom of the tank than at the top. Try to make all drilled holes near the top of the glass sides where less pressure is exerted.

 

Same concept applies, physics is physics with regards to fluid dynamics. Obviously not going tonus a diamond drill bit though.

 

Sent too soon. You will want to use a carbide or wood hole saw, and go slow.you want to scrape layers away and not melt it.

 

The crackling on the edges as in crazing? Cloudy spidering to the seams? Map gas is a big nono. Burns way to hot. I use propane mini torch for all of my acrylic ato holders and any custom work I need to do.

The splitting of a seam is a bit different though. Heating it up may force a bit of a bond. But chances are you are going to make the joint more brittle by heating it up. Acrylic is not glued together so much as it is welded chemicaly. You would probably be better off trying to reseal it with a bonding agent. Weldon would be a good start.

I would like to note I have limited experience with aged acrylic. Most of what I have worked with has been new
And I retuned my sumps with some simple cyanoacrylate.

 

That sounds like crazing from the drilling. the question is going to be how close to the top of the tank they are. I guess the best test aside from extensive calculations would be to fill it up in the front yard and pray. The crazing could be surface splitting from drilling too fast, and could be innocuous. Or it could be pressure shearing and splitting from the bolts. Which could result in catastrophic failure. It all depends...

I have no solid answer for ya. Maybe take a pic?