specific gravity for nerite egg hatch?

just wondering at what specific gravity nerite snail eggs will develop? have there are 5 in my brackish planted tank 2 of wich are always laying eggs. my dwarf puffers tend to eat most of the eggs but none of the ones they miss seem to hatch. the tank has been set up for close to 8 years if moving it twice does not count. i added the nerites over a year ago 2 greens 2 tiger(rosy/tracked) and one zebra