Solutions for tap water deadly to inverts?
I have no trouble keeping community fish.
I have no trouble keeping most plants, barring BBA issues.
Yet, even with tapwater at pretty low KH and GH - both at 3-4 degrees or so, I've been unable to keep inverts alive. Pond and ramshorn snails don't get bigger than 1/4" or so, and the few shrimp I've tried (only ghost and yellow shrimp) die off impressively fast. Even nerites don't last more than a couple months.
I have used a GH booster regularly, so it doesn't seem likely that Ca, Mg, or Mn deficiencies have been an issue. Most of my tanks stay in the high 70's, and the one yellow shrimp-only tank (10g) I tried was kept in the low 70's.
So I have two questions. One, is there anything I'm missing, or should test for?
And, two, and this one is probably the more important of the two, are there any especially cheap R/O units out there? With a single 10g tank, I might only need 1-2g or so of R/O water per week, so a $200 unit would be completely out of the question. Any ideas? I'm not looking to keep BKK's or SSS+'s. Just a shrimp/snail friendly tank.