Light color on head and spine

[David Ellsworth]

I have some new Rainbow Shiners in quarantine, they arrived 9/23, so I've had them about 7 days. I noticed a light color spot on the heads of several fish and it travels down the spine of the fish. I don't see any spots or discoloring anywhere else on the fish. I have never kept this fish before and I wonder if this is natural for this fish. 

I tested my water and found about 0.5 ppm ammonia in it and did a water change of about 50%. I didn't see a large drop in ammonia so I tested my RO water and it had a similar level of ammonia in it. I dumped all that RO water and made some RO/DI water and remineralized it with a GH/KH Shrimp King shrimp salt. This RO/DI water lowered my ammonia down to 0.25 ppm in the quarantine tank after a second 50% water change. I'll change some again tomorrow so ammonia is reduced further.  Water perimeters are:

PH: 7.83

GH: 9

KH: 3

Temp: 71.4 F (cold water fish so no heater)

Ammonia: 0.25 (after 50% water change)

Nitrite: 0

Nitrate: 0

[Patrick M. Bodega Aquatics]

Maybe stress from the stress of being in a new aquarium? My fish always lighten up when stressed.