My 2 Green Spotted Puffers will sometimes turn a greyish black color below their pattern, where they would normally be white.  At first I thought it had to do with the nitrates being over 10ppm, but religious WC's haven't changed anything.  They dont act wierd or stressed & they eat just fine.  I am in the process of raising the salinity in that tank, but I stopped adding salt as soon as I saw this going on.  Their salinity level now is about 1.008.  I was going to take them to 1.012.  Should I be worried?  Any advice?