Looking for help drilling a biocube

What type of hole saw are you using & are you placing putty around the drill hole to pool water? Putting duct tape on the inside can help to stop the glass from chipping when your hole finally exits the other side.

 

Could you post a photo of your cracked tank?

 

Once a crack starts it WILL continue to grow until it terminates at an edge (or other crack). I would recommend:
1. 600 grit wet dry sand paper get it wet and work the hole till smooth. This removes imperfections that could start a crack. Like large saw blades that are cut from the outside edge in for heat expansion, a circular hole is drilled to prevent a crack from occurring from the square edge. If the glass has a tiney square edge there, its the weak point. Test how smooth the hole is by dragging your nail perpendicular to the edge. Once its decent check smoothness with finger. Sanding is probably is not normally done however with it being thinner & weaker glass the imperfections will be much more susceptable.

2. Go to Menard's and buy a nipple for vinyl tubing. Menard's carries ones that are elbow with one side threaded. Run vinyl from the bulkhead for around a minimum 6". This will allow flexing between the ridged pipe and the solid pvc.

 

Another idea: It might work to take two pieces of 1/4" acrylic drilled same hole size as glass. Sandwich the glass, silicone between glass and acrylic on gasket side of bulkhead. This would distribute the face flex load on the glass. I'd still sand the hole.

I'd recommend starting with my first suggestions.

 

I've used JB clear weld to stick acrylic to glass with great success. It takes a chisel to bust it off! Don't use silicone. You also have to get the gloss finish off the acrylic with sandpaper before you can bond / glue it to anything.

 

Additionally I suggest using lexan rather than regular acrylic for this purpose. Lexan is very difficult to crack.

 

And abower is right about using tubing for the first 6 inches.