What salinity or specific gravity are you bringing the water up to? The species you've got (looks like Pomacea diffusa) usually considered incompatable with fish that need elevated salt content in the water beyond about 0.004sg.
The nitrate is rather high for a tank with snails, although if the reading is accruate it's probably not the culprit. I don't know the range on the kits you're using, but a lot of test kits I've used over the years only have accuracy to close to 40, so readings that appear to be 40 could be >40 on those kits. The amount of copper is small in that fertilizer, so one or two doses I doubt would be the culprit. However, if it's been dosed regularly for a while and the copper concentration is not monitored, that's the sort of thing that would be worth checking on if you plan on keeping more inverts in the tank. Some LFSs will test copper, although the bigger chains tend not to. If you're going to be adding a fertilizer that contains copper and can find a copper test kit for freshwater, that would not be a bad thing to have on hand so you can keep tabs on the concentration in the tank.
Since you were adding a calcium supplement that from my memory is also carbonate based, and the erosion doesn't look like it progressed
too fast given how many months went past between the first and last in the pics, it could just be normal wear & tear showing. kH would be worth checking on, but with regular supplements containing CaCO3 it's not likely to be the issue. It's not uncommon even in good water for cases of shell erosion on old growth to be in a more-or-less constant state of gradual decay, since shells do dissolve slowly. Once the protective protein layer is gone and the snails will be constantly redepositing new shell or calcifying tissue as the shell dissolves away very slowly. When snails are transported from one set of conditions into another that is different enough (particularly between extremes of pH), the growth created in the old environment is often subject to erosion under normally acceptable conditions moreso than the new growth.
The shells all show two pronounced growth lines, one about halfway back on the outer whorle and one near the front. Even if the snails were gotten when they were the size indicated by the growth ring further back, that would probably put them at adulthood and mean that you'd had them for a number of months if they were all gotten around the same time. Bearing in mind that P. diffusa isn't especially long-lived in captivity, you may not be seeing anything other than life expectency. 1-1.5 year lifespan is normal for a lot of P. diffusa, especially those kept at higher temperatures. Those in colder temperatures may last 2-3 years, although it's not all that common.
Also, what do you guys do with the shell once your snails die?? kinda feel weird keeping them around lol
I save them when possible; over the years I've accumulated several drawer-fulls of them. I keep them around in part for sentimental reasons but also for the purposes of maintaining a shell collection, since I study these snails somewhat informally on the side of my other work/studies. If you hang onto your shells, you might find use for them in the future. If they're in good enough shape, they're usefull for animals like hermit crabs, but can also be a useful thing to have around if another similarly sized snail experiences a sever shell break and requires a patch job.
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