red outline around pattern of undershell.

by Vince
(Miami)

Hey. I have 2 red eared turtles. They are around 6-7 years old, and around 7 inches long. I live in miami, ver hot weather. They get their water changed daily. About a week ago I found like a red outline on the pattern of the undershell, im almost sure its blood. Theres also like a stain that you can see under the layers. The big one has it a bit worse than the smaller one though. I've been trying to do my research and find a solution for this but I just cant seem to find anything so your help will be greatly appreciated. Please tell me how can I cure this. Maybe a diet change? Lack of vitamins? Need of additive in the water? Thanks!