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So shells would actually be good for swordtails too? Though not sure what kind of water hoplos like, I think they're related to cories so that would be a problem with placing shells.It is really simple ICEAL. Sea shells are made almost completely from calcium carbonate. In any tank that will raise the pH somewhat and will increase hardness and KH by some amount. Mollies love water high in minerals and will respond positively to those chemical changes but many fish, like tetras for instance, will be stressed by the increased minerals and KH. All "rules" really depend mostly on the fish that you keep. If you keep common livebearers, like you can find at a typical fish shop, the shells will improve things for your fish. If you keep tetras, Corydoras or angels and similar fish, the shells will give a negative impact. The only rules I know of that can be applied consistently to all fish is to keep ammonia and nitrite levels at a minimum and to feed the correct amount of the correct food daily. Even defining the correct food is one of those things that changes depending on what fish you keep. A typical predator does best on a very high protein diet while something like otocinclus will completely ignore anything but actively growing algae.
I cannot find much info on their water preference, all it says is that it adapts to a wide range of water stats, even living in polluted areas, but no mentions of hardness preference or salt resistance and such.If the haplos like soft water, not something that I know at all, then the shells would not be a good thing for them. As you have surmised, swordtails, especially the swordtail/platy crosses sold in most shops as swordtails, would find molly water just perfect for them.
My tank will probably always be in the formation of swordtails + 1 hoplo. I don't intend on breeding hoplos as that would need too much space, one kitty is enough.If hoplos are easily adapted to different water conditions then they should at least be able to survive in molly water. Breeding may be quite another story. Hard water, which you will be creating, can be very difficult to overcome if you want to breed any of the more common egg layers. It seems that the egg itself becomes almost impenetrable to milt when there is too much calcium present for many egg layers. On the other hand, after an egg has hatched, often the fry do just fine in harder water.