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Your tank will have to be cut out of the boat. Seating and access covers removed and the floor cut out for complete access. This is a job for a professional fiberglass shop.
Yes aluminum tanks can corrode. They corrode from any water that is allowed to pool or is trapped between the tank and the supporting structure/foam. Over a long period of time they will corrode and develop leaks.
If fuel with ethanol is used the ethanol attracts water, condensation. Then this water and ethanol combination combines with other chemicals in the fuel and creates a mild acid. This settles to the bottom of the tank and corrodes the aluminum. This separation is called phase separation. It is becomming a big problem in the marine industry.
Replacement of the tank with a poly tank is the only option.
I would have the tank pressure tested before trying to have it removed on the chance it is a fitting or hose leaking.
Not a good position to be in. Sorry. Let me know if I can help further.