Maybe you have some sort of snail killing parasite in the 2 tanks.,...and
not in the third..?

Allen Smith wrote:

The only reason that I think that the tanks are Ca or Mg deficient was
that the plants were showing signs of that in the 72g, but I wasn't
sure which it was and was hard to diagnose. For a period of several
months I added 1 tablespoon of epson salts which is enough to raise
the Mg to a high enough level if that was the problem (for the
plants), but that did not solve it. That led me to believe that Ca was
the problem and I haven't addressed it yet, as I can't find an
economical and controlled way to dose it.

I will test every tank tonight or tomorrow for pH, gH, and kH and
report back to this thread.

Thanks for all the help.