Then disaster struck..LOL...my plumber buddy showed up yesterday about 9 AM, we started running copper and gazing at the situation not 100% certain of what had to be done. About noon we where all tied in, pressurized the system, no leaks...THOUGHT we where home free, while bleeding the system we used the water to begin filling the tank, inch by inch, the tank held tough. Half way up, it's about 5 o'clock by now, dinner time. We have a quick meal, go back downstairs, 3/4 full, looking good, almost at the top...Tank Leak...panick mode., 2 hours later tank drained down, about 15 gallons hit the floor, shop vac, nightmare...finished cleanup about 9, passed out about 9:15. Woke up regrouped, short version , the entire tank has a steel frame welded, except the Bottom. The bottom was built out of wood sandwiched 1-1/2" foam, solid but wood, I created a lip that the walls slid into, then screwed from the sides.. Then concrete floor, coating, etc...I think as the weight of 400 gallons started to bear down, very slight settling / compression, occurred in the base, this settling caused a very slight separation between the floor and walls..i have to remove the exchangers, get in, and take a close look, from the top everything looks perfect, but there has to be a crack, it was leaking out pretty good..What to do now, because I hated the look of just shoving in a EPDM liner, I opted for a cement based coating, the coating did not have enough elasticity...I have two options as I see it, but I am open to suggestions. There are a few liquid EPDM products out there, this would be the easiest..Coat and go, there are also a few companies that will weld up a custom EPDM liner to fit the tank...not sure which way to go yet, the custom liner sounds safer..I am tired but not beaten, Happy Holidays, it's going to be near 70, who needs storage anyway..LOL