DIY
if its just the fitting that it leaks from at the outlet, then take it out and put loads of ptfe tape around it. If its from the inset metal part of the outlet attached to the tank or a crack or something, you might be able to use fiberglass resin. Sold at focus or somewhere, garden center.. takes about 2 days to dry properly, use some fiberglass cloth if its a larger crack, the resin will bond to the plastic.