Fuel isn't good for balsa but as long as you didn't let it soak in you should be OK. The big thing is you need to find out where it came from and fix it. First guess would be that you may not have gotten the stopper in the tank tight enough. Second guess is that it may have been too tight [:@] and split the tank. either way the leak source needs to be found and fixed. If possible you can try to coat the tank area (the soaked area) with a layer of epoxy to seal it from the fuel in the future. Be sure it is clean/dry first.

BTW - didn't I tell you it would be a lot faster than your previous planes? Glad it all went well.