If its a polyethylene tank as most are your options are limited,hardly anything will stick to it.I did alot of reading on the subject and eventually found a loctite product at the local ace hardware that is made specifically for polyethylene and other hard to glue plastics,it is a 2 step system first step is a sort of felt tip pen that applies the primer second is a liquid in a very small tube that is the actual glue.I cut a small piece from a coffee container and glued it over the pin hole leak that i had.
The other option is plastic welding,lot of instruction on that subject on U-Tube.
Having spent many years in the boat repair business i thought 3M 5200 would stick to anything so i tried it in a different spot,let it cure for 7days as the instructions said and seven days later it peeled off with very little effort.
This is only temporary unless it works! Red Green