How broken and where is it broken? If it's a cracked side you can get away with the sandwich method that I've used that works just fine, provided you don't mind a little ugly.
I measured the inside and the outside measurements, of the glass and went to a glass shop by me. After I told them what I was doing, they had some extra safety glass pieces sitting around that they cut down for me. Not sure if that makes that much of a difference for a 30, but it can't hurt.
i also picked up two tubes of glass epoxy. (It's stuff they use to patch chips in windshields and big caulk gun sized tube of 100% silicone. It has to be 100% silicone too. No anti-fungals no extra anything since it's the extra crap that can be harmful to the fish. It's a bit tricky to find, but I've actually seen it once at Walmart as their "economy" window seal.
Then, I cleaned out the cracked glass area with vinegar and then rubbed it down with alcohol wipes. The guys at the glass shop told me the alcohol trick. Then I set up the glass epoxy in all the cracks. I did one side let it set overnight then did the other side.
After the epoxy set, I took the outside piece of safety glass and siliconed it flush to the cracked original glass on the outside and then did the same on the inside, making an epoxied glass sandwich. That set for however long it took to for the silicone to harden and after that I tested it out to make sure it didn't leak, and then put it back to use.
This was either last summer or two summers ago now, I can't remember for sure. Once it was done it was filled to the top with fish, but since the beginning of summer, I switched out that tank for some small turtles.
It has a definite lean to one side when I go to pick it up, due to the weight of the extra glass, and I made a bit of mess being heavy handed with the epoxy on the first side I did, but the side I patched is up against the wall here and with a few plants or in my case currently a turtle dock, you'd have to really squint at it to know it's there.