As requested I have retested the pH, gH, and kH in every tank. The
test was performed twice to ensure the result was correct, and a
second person would examine the results to see they agree'd with my
color interpretation.

Here are the measurements and the recap of the inhabitants of each
tank:

72g pH 6.9 gH 8 kH 4
Inhabitants: 30 cherry barbs, 3 white clouds, 2 misc. danios, 6 kuhli
loaches, 1 pearl gourami, ~6 SAES, ~5 ottos, 1 clown pleco
Snails do not survive in this tank. As a sidenote, I should have said
I had one ramshorn snail that lived for about a month before dying,
twice as long as any MTS.
Tank is hi-tech setup with lots of light and heavily planted.
Water change frequency of 50% refill then change of 50% again 1x
weekly.

55g pH 6.6 gH 4 kH 1
Inhabitants: 1 6" oscar, 1 8" sailfin pleco
Snails usually disappear and aren't seen again, if I find them their
is just a shell left; not crushed or broken.
Water change frequency of 50% 2x weekly.

20g pH 6.8 gH 4 kH 1
Inhabitants: 1 large angelfish, 2 misc rescued tetras (flame and
rummynose I believe)
Snails thrive in this tank. Tank is lightly planted with crypts and
apon. crispus, no fertilization.
Water change frequency of 25% 1x monthly.

So... This tells me no reason why the snails are not surviving in the
other tanks. What exactly does everyone else think? Are the kuhli
loaches the real culprit, I cannot imagine it is them, but I cannot
narrow it down to anything, a gH of 8 seems high enough, does it not?
In the 55g, the oscar does not ever bother the snails, so why do they
die? The gH is 4 the same as the 20 gallon and the MTS thrive in the
20g.

I'm boggled and have been trying to solve this for over a year.

Some people may be shocked at such a low kH (myself included) but the
fish in those tanks have been living like that for 2-3 years very
happily. No sickness ever has happened, or wild pH change that I know
of. Consistent water changes deals with the low kH IMHO. But I may
start adding some sodium bicarbonate (to raise the kH to at least 3)
as a precation, if the rest of the group feels like it's a good idea.

In respond to parasites, all the fish have been quarantined properly
and treated with flagyl and fluke tabs whenever I get them. So unless
there is something that gets by that, then I don't see diseases or
parasites as a reason for them dying.

Help would be appreciated. Anyone smart or ingenious out there that
can solve this problem?

Very much appreciated,
Allen Smith