Ahh, so you're not asking for a alternative technique. You were more concerned whether your process will end up softening your water or not or why it's happening in the first place and keeps coming back. Well, I've seen all sorts of answers for that one. From type of fish food to fish poop and not cleaning filters. Perhaps it well could be the slimy snails.
Hmm. What if you net it? I suppose that's a little like skimming so you're not getting rid of it.
I suppose you could look into boosting back whatever hardness you soften with gh booster but now you're getting into adding "stuff" when you're trying to keep things pure. Actually, since you're testing hardness, have you seen any change since you started your jar removal and filtered water add? By the way, since we're throwing out ideas, I don't know how you filtered your drinking water but I recently got a TDS meter and was playing with it with water filtered by my PUR filter. It didn't seem to significantly reduce any TDS from tap. Sure, the PUR filter has carbon and other media to get rid of chlorine but one does wonder what its missing. I tried the one from my fridge and it cut half the TDS so maybe its a matter of needing to change but still it was in the 100s. I think this is the selling point of Zerowater filters. This is also probably why the serious shrimpers go out and buy an RO system.
Interesting conundrum. I hope someone here pipes up with something cool or you find a slick solution.