disease question- black color between scales

Question
I have had one of my planted tanks running for about 7 years now. The plants have matured really beautifully and it has been doing well as a system. Everything is really splendid except for one thing, the fish. I keep having problems with some disease that I just can't get rid of. It could be the store or it might be in my tank, but every time I get fish they have come down with the same symptoms and usually die a few days later. The fish will develop a black pigment that shows up in between lines (like someone took a pen and drew outlines around every scale). Some fish last a long time, a few lasted almost a year, others die within 24 hrs. Everything in the tank is fine, temp pH nitrates etc. But the fish keep getting this same disease and I am wondering if you have a diagnosis or treatment. It has gotten to the point where I'm not going to buy fish anymore because they just keep dying. I really don't want to tear down the tank, I'll just run it without fish if I have to, but if there is anything I can do please let me know. Thanks in advance!

Answer
Since this is a long standing issue with you tank, I am incline to think it is something in your tank or your water.  You could have high levels of chlorine or heavy metals.  I ma not sure what type of fish you are buying, but you could try some super hardy fish, like zebra danios.  I can't find any information on the specific symptoms you are describing, but most things that kill fish in a 24 hour period are water related.  You could also try buying bottled water (if it's a small tank) and see if that makes a difference.  It's odd that plants would survive in an environment that doesn't support fish.  Please keep me posted on this, I would love to know if you figure this out.  Sorry I couldn't help more.