Another dwarf neon rainbow not doing well - discolored spot?

[Schlteacher]

Sorry to keep posting, but just wondering if anyone with more expertise might know what the problem is - I posted several days ago about a new dwarf neon rainbowfish not doing well, and now another one seems to be going the same way - hanging out near the surface, seems weak. I noticed what seems to be a discolored spot on its back and am wondering if that might help to figure out what's going on? I haven't had this type of fish before and don't know how they normally look, but the other ones don't look like that.

Tank parameters: Ammonia 0, nitrites 0, nitrates 10, temp 78, pH 8.2. 45 gallon tank set up about a month; did fishless cycle using old media and began adding fish last week. Aquaclear 70 filter. Other fish: 8 cherry barbs, 1 angelfish, 4 other dwarf neon rainbows, all doing well.

 

[ZxC]

praecox rainbows have poor genetic strains and are almost always doomed from day 1, you are experiencing that and there isn't much you can do to stop their slow wither away.

 

[Schlteacher]

Well that's too bad. I just lost two more now. Is it really just genetics and not something I'm doing wrong? I did everything by the book with fishless cycling etc and my numbers are all good, but it just seems strange that they're going so fast.