[Pumilo]

I own a glass company. Need some more info though.
1) I don't understand what you mean by a corner "L" cut
2) How thick is the glass?
3) How close are your lights to the glass?
4) Are you sure the top of your tank/viv is nice and flat? No small piece of sand or something in the way?

 

[Pumilo]

Still can't quite picture your L cuts. A photo might help. But any decorative cutting is going to make it weaker. Especially when you add some heat from the lights. It will expand and contract funny.
Fish tanks generally use a plastic strip along the back to cut out odd shapes for tubes, cords, and the like. That way they avoid the odd cuts.
If there is no other way, consider having one tempered for strength. That will cost you though.

 

[Pumilo]

Edge stress concentration! Instead of an L, just get the corner cut at a 45.

Probably cheaper that way too. If your L cut is what I'm thinking, that's a bit of a trick and probably half the price.

 

[Pumilo]

I wish you could have gotten a picture of what you are trying to accomplish here. It sure sounds like you are spending way too much for what should be a simple solution. We can only help so much when we don't really know exactly what you are doing.

 

[Pumilo]

Contraction from being cold, followed by expansion from the heat from the low lights, combined with weird funky cutouts that are not used on glass tops for good reason, equals cracked glass.
Hopefully, being tempered, or heat treated, will help the problem. But I don't believe that it is the best solution.
Your potential problem now will be that if it doesn't fix the problem, and it "cracks" again, it won't just crack, it will break up into several hundred very small pieces, scattered all over the inside of your viv.
I sincerely hope it fixes your problem and I'm sorry you were unable to get pictures so we could help you find the right solution.

 

[Pumilo]

You have to use your imagination for your particular light fixture and application. It could be hung from chains/ropes, spaced up with PVC spacers or wood blocks.

 

[Pumilo]

That distance should be a lot better. Fingers crossed for you!

 

[Pumilo]

I don't know what else to tell you. They weird cutouts you feel you need to do, make it fragile and iffy. You chose to go tempered instead of providing a diagram or photograph of what you were trying to accomplish.
Now you wait and see if it's going to do the job for you.
My honest opinion: find a way to do it with a simple, standard, rectangle of glass, with possibly one simple corner cut off for cords.