Godspeed wrote:
Anyone know how to fix a leaking 5000 gallon steel water tank on a
concrete pad?

One of two tanks is leaking from the bottom. Not much, but enough to
keep the concrete always wet (which likely is rusting it out even
more).

I can't figure out how one would go about fixing this kind of leak.

Not enough information. Chances are, the tank is rusted, very thin,
difficult to fix. It may be possible to weld a plate over the area, it may
be possible to line the tank. If it is already lined, you cannot weld on it.

Inspection is needed to answer your question. There are companies that
specialize in tank repair. Call one to look at it.