On Monday, March 12, 2018 at 1:14:07 PM UTC-4, Mary M Piazza wrote:
the tank is full & gauge drops down to empty

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So, take it out and find out what's wrong. Again, no useful info.
This is like saying my car won't go, what do I do? Is it a simple
275 gallon tank with gauge on the top or a buried tank with some
kind of gauge for that? If it's a 275 gallon tank with the gauge
on top, it can be easily removed.