This oscar at least, is very docile, the sailfin pleco is about 2-3"
larger than him and routinely he tries to bite it, but can't bite
through his skin and the pleco just ignores him. If the oscar is
bothersome enough he'll get a good whap on the side from the pleco
which makes him go hide. So overall, he isn't too much.

I do not find shattered or broken remains of the shells. I find empty
shells that have usually (but not always) whitened slightly. I don't
know if this is just due to decalficiation overtime of if they have
become calcium deficient and cannot repair their own shells and die
overtime as a result. It seems like concerning my water parameters
which I stated in my first message.

I use plain well water which comes from a well for the 55g tank. pH
7.2, gH 3, kH 5. The pH as it is well water can vary, but usually it
doesn't drop below 6.8 and is never higher than 7.2, the gH and kH
always stay the same. Water changes are done 2x weekly with 50%
changed if that is useful information.

What is confusing about all this is the tank the snails do flourish a
20 gallon with the same water parameters. Inhabitants consist of one
large angelfish and 2 varius tetras (rescued fish). As far as I can
tell they leave the snails alone and the snails flourish quite well in
that tank as the tank is the source of all my experiment snails (I
pull about 50 extra snails out of the tank each month at night and
dump them into either the 55 or 72).

In the 72g (same water, but with substrate of onyx sand that
supposedly raises hardness - did the first few months, but doesn't
now) I have tried raising the hardness by using 1blsp of Epson Salts
after each water changed, it did not seem to have any effect, so I
don't think they are dying because of a Mg defiency.

If I negleted to mention the inhabitants of the 72g here they a ~
30 cherry barbs (they keep multiplying every month), 3 white cloud
minnows, 2 assorted danios, 1 pearl gouramie, 1 clown pleco, ~4-5
SAES, ~6 ottos, and ~8 kuhli loaches (6 black variety, 2 normal).
Contrantry to what people keep recommended I do not believe it is the
loaches in this tank that are stopping the MTS from thriving - I think
it is the water hardness, even though it is no different from the 20
gallion. That is why I don't get it. So it could possibly be the
loaches, but I copiously feed the fish in that tank, I doubt the
loaches are hungry to go after such a tough shelled individual - they
certainly wouldn't be able to crack open an adult shell the only
possibility is that they might be able to swallow a (very small) baby
snail whole perhaps, stopping the MTS from reproducing? I find the MTS
shells on the surface of the sand, with a much whitened appearance
compared to how they were when they were first put in the tank (a dark
brown, almost black color), leading me to believe it is water hardness
issues.

All 3 tanks test the same for gH and kH, and that is why it is
boggling to me as to if the problem is water hardness why is the 20g
not effected and why are the snails doing well.

I have not tried Ca addition as I don't know of a controlled way to do
it that is also cheap. Is there a formula I can use to drop in 1
tablet of Ca & Mg (vitamin for human consumption) to figure out how
much the water hardness would raise. I have tablets that taken as 3
provide 1000mg Ca, 400mg Mg and 15 mg Zinc (If Zinc is a problem I can
obtain quick dissolving pills without Zinc - but I haven't really
heard anything about negative or positive relating to fish and 15mg
seems like a very small amount - but I may be wrong). It seems like a
likely idea to use them to raise water hardness, but I haven't heard
the idea proposed before. I dislike the idea of using seashells or
eggshells as it is hard to control the water hardness that way, and it
involves much more experimentation and constant testing to ensure that
you are not raising the hardness too much. If I could find an easy way
to drop a certain # of tablets into a tank (per 10 gallon) after each
water change, that would be immensily easier. Any chemists or
extremely smart people up for the challenge of telling me how to do
that? I'm certainly not intelligent enough and may just end up using 1
tablet after each water change and increasing it 1 tablet at a time
and contually testing the water - but I don't want to do that if
someone has a more accurate idea on the dosage I could use.

The other alternative is to dosing by human vitamins is to purchase
Seachem Equilibrium, but I have worked out the cost to water change
ratio and it is not economical for me to use that product, thus I'm
looking for other ideas and alternatives.