TEst your ROwater.
Also, check your test kit. I had a cheapo test kit for nitrates. My trates were always at 10ppm, but my inverts were dying, so I took a sample to the LFS for them to test.
75ppm is what it turned out that I had. Let me tell you, I bought a reliable test kit after that!
But if you are putting in very acidic water, that my be the problem