Some times it is hard to determine what the tank is actually made of. I would get some MEK and dab some on both the tank and pipe if the solvent softens the plastic material the Plastic Mend or other solvent based adhesives will work, the solvent with partially dissolve the plastic then flash off leaving a good chemical bond. If the tank is made from polyethylene the solvent based adhesives wont work, there you need a mechanical bond and need more bonding surface so a wide flange as someone else suggested is a good idea.
