there are afew things that might work. how ever you say the crack is on the bottom of one side of the tank, thats a bad spot since the pressure exerted by the water is worse at the bottom then the top, I wouldn't see the fix being premanant at all but merly buy some time and since if I read that correct it's on the inside of the tank? you'd have to empty the tank cause you would want to get all the water out of the crack before fixing it.
that said you could simply useing a hair dryer to evaporate the water out of the crack then use some super glue (not the gel kind it's to thick) and let it dry useing a uv light or in the sun on a hot day then use a razor blade to skim off the excess, this is a trick if you have a small crack or chip in your cars windshield and alot cheaper then having a pro fix it, when it's basicly the same process and stuff they use.
after useing the super glue I would then seal it with some aquarium safe glass epoxy like that which is used to build the tank s in the first place. this way the water doesn't come in contact with the glue and it should be safe for the fish but to empty a 90gal, man thats alot of work.
be safe get a new tank or have a pro replace that pane/side of tank