Repair Tape

Many have mentioned a repair tape they keep in their tool box for emergency repairs of things like hyd hose, fuel line etc. Don't remember and can't find in search of "repair tape" what the brand name and good source for this tape would be. I want to repair a small gas leak in my garden tractor fuel tank - plastic tank with a "welded" center seam that is leaking when you fill the tank. Been just filling below that level but would like to repair w/o having to purchase a new tank. If you could provide the name and source for this, I would really appreciate it.