looks can be deceiving though
My aluminum tank looked good too, then some diesel starting appearing in the bilge. Of course I thought it was coming from somewhere in the fuel system or hoses, but as I found out during half way across a 3hr motoring, it was the tank. I went down below to check something and could smell the diesel. The whole bilge was filled. I managed to push my boat back to my Marina, which just happened to be close at the time, with my dinghy strapped to my swim platform for about 4 hours right into the lift. I managed to save just enough fuel to complete the haul out. After draining the 50 liters of diesel and getting her back on the hard, I removed the tank in an hour by cutting out some of the wall between the locker and aft cabin. That's when I found a hole the size of a dime in the bottom, forward port side corner. It had been rubbing on the stringer for 30 years. We have a great welder in town and he patched the hole and refurbished the whole tank like new for only $100CDN!!!! I questioned him on replacing it, but he assured me it was like new, and that the diesel trucks and buses don't just throw their tanks away when in need of repair
So if possible with a mechanics camera or such, check out the underbelly and corners of the tank....you never know?