Oh crap! I cracked my sump!
 

I was trying to get that extra quarter turn on a fitting on the bulk head and cracked it from the bulk head hole toward the top corner of the sump. It's not leaking and the crack did not go all the way to the edge of glass. Will it eventually begin to leak? Is there anything I can do to stop it from going all the way to the edge? The crack is on the end piece of glass on the sump.

That really sucks dude,I had the same thing happen to me on the 75 corner I had. It only made it a week before it started to leak and had to replace the pane of glass.

I would silicone a piece of glass or acrylic over the crack.

Glass would be the best, if you use acrylic make sure you use some PVC primer on the edges. I would also put it on the inside of the sump to hold the pressure off the crack.

Yup, I've repaired cracks below the waterline successfully like this.

if you can, silicone another piece of glass from the inside of the tank using silicone. Make sure you put a nice bead around the edge of the glass to give it a good seal.

If your sump is glass, repair is with silicone and glass like said above. If your sump is acrylic, use water thin methyl chloride and acrylic to patch.

It is a glass sump. Should I lay a bead of silicone around the inside edge the glass and push it against the cracked piece so that the silicone is pushed out around the edges, or do I set the patch glass against the cracked piece and silicone around the edges? The cracked piece is a location where it can not be clamped to dry.

Put a bead directly over crack on sump, then zigzag on the patch piece/around edge. Press patch piece on, tape it in place untill it sets, an hr or so, then remove tape and apply a bead all the way around the patch edge

It doesn't look pretty, but will last.