Here's something to try. I have repaired tanks in the past - all successful. Take another old tank (if you have one) and cut a strip from it about 1/4" wide x the length of the tank. If you don't have an old tank, a strip from an old plastic milkbottle will work. Simply light the strip with a match, while holding it over the hole in the tank to be repaired. As the strip burns, it will drip red-hot drops of plastic which will weld themselves to the tank, and make the repair. Naturally - this needs to be done outside, as the burning plastic will emit toxic fumes. Let us know how you make out.