Neritinae In Danger?

[maurizio]

Hello all,
brackish, SG 1.002 @ 25 C, one year old. Got the snails in January.

I recently saw algae on the glass, but I assumed it was the result of the recent addition of fertilizers. Only a few days ago I realized that the snails were not eating at all, and actually they were nowhere to be seen. The only one visible one remained in the same spot, on the sand, for some three days, before I decided to check it out, and realized it was dead. I had then to physically search for the others, which were hidden in the most secluded corners of the mangrove roots, not moving. I took them all out, put them on the sand at the tank front and watched them quicly move away, and hide again. Only one spent some time on the glass, eating a bit before disappearing. What's happening? To my knowledge, no chemicals have entered the tank, except the fertilizers I've put in myself. There may be an issue with NO3 and NO2, though: the Tetra stripes say everything is fine, the liquid tests show too low values of nitrates (15 mg/l, lower than tap water), and apparently 0.05 mg/l of NO2.

Any help?

 

[maurizio]

I can add that they are clearly staying away from light, at the moment: I have the impression that they come out when lights go off.
I finally have a final reading of NO3: ~75 mg/l. I know, it's a lot, but it's not unusual in my tank, with a tap water starting at 40 mg/l...

Anyone?