checkerboard cichlid question

I've researched on the internet and can't find a clear answer to these questions, so if anybody has any ideas...

Background and setup info:
My friend has 4 checkerboard cichlids (D. Filamentosa -- sp?) in a tank with several discus, gold tetras, siddhamunthis (sp? -- small loaches) and otos. He uses some liquid fertilizer, most plants do not do well at all -- lots of algae on most, or they melt away. He's pretty much limited to tiger lotus at this point, but they do grow like crazy. He keeps the water extremely soft, like 2 GH max ... 40% water changes weekly ... nirates (NO3) typically around 15 ppm. He injects CO2 ... pH typically around 6.1.

The tank has been up and running for about a year. All fish are doing well, except for the checkerboards. They have slowly, but surely, been deteriorating. Their fins are basically melting. I'm almost certain this is not a fin rot fungus, as I've seen that before. This is much, much slower ... just seems some condition isn't right. One chat room recommended to keep NO3 below 5 ppm for checkerboards. The checkerboards eat well and even seem happy (without fins), but I'm sure they will die soon if something doesn't change.

Anyway, bottom line is, he is going to give me 2 of the checkerboards to see if they do better in one of my tanks. Fish do well in my tanks and I have helped several fish recover in the past. If the NO3 is a problem, one of my non-CO2 tanks will easily go to 0 ppm NO3 if I allow it to do so. Therefore, I can just maintain that one at 3ppm or so, so my plants get at least a little NO3! I'm a little concerned that my hardness is about 10GH in my tank. If these checkerboards were wild caught, that may be too high. My pH is just over 7, also high for checkerboards probably.

If anybody knows checkerboards, please let me know if you have any ideas.

Sorry for the long post. Thank you for your patience.