I never really found out what was going on. The leak was getting faster, so I had to take action. I set up a spare 10g I had laying around, moved some of the sand from Jack's tank to the 10g, moved the filters (450gph filtration on a 10g!) and heater, added all new water, and moved the fish. Even though I had been doing aggressive water changes on his tank and using carbon in the filter, the 90% or so water change from moving him (there was still water in the canister filter and the big HOB, that's why I figure it was about a 90% change rather than 100%) seemed to do the trick. He's been acting pretty normally in the 10g.
The 29g is now mostly emptied except for a tiny bit of water and the remaining sand. I'm going to make that leak work for me to empty the tank out, along with my good buddy Mr. Evaporation. After it's completely dry I'll take all of the sand out, clean the tank like nobody's business with bleach, and fix that seal. Should be no problem to move the sand back, move the decor back, and put the filters back on the old tank. I really hope the repairs work, as I had plans for the tank after moving Jack into his new bigger home in the spring.