On March 18th, 2013, I have posted about this same problem although its getting worse. My turtle has black spots on his shell and im not sure on what to do about it. There are no marks on the bottom like these and it won't just brush off with a toothbrush either.
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How big is your turtle? Just a baby with about 2 inch shell length
How long have you had it? Since about November (5 months-ish)
What is the water temperature? 75
Did you use a thermometer? Yes
Are you using a water heater? No, my house in generally hot
How much water is in there? 3-4 inches high
Are you using a water conditioner? I use calcium blocks which also condition the water
Are you using any filtration? Yes
What is the basking temperature? 85
Is there a basking light? Yes
Is there a basking platform that is easy to climb on? Yes
What kind is it or what is it made out of? One is made out of rocks and ones a plastic platform I bought from the pet store for them
Is there a UVB light? Yes
What have you been trying to feed it? I feed them regular turtle pellets and sometime things like lettuce as well
When was the last time your turtle ate? Today, both my turtles eat well
How big is the tank/pond/enclosure? Not sure; Its a 55 gallon turtle breeding tank I bought used from a local pet store
Is the tank near a window? Not really but is in a small room like 7 ft away from one
Is the tank in a room with a lot of activity? No
Have you read the Basic Care section? Yes
Have you searched the forums for similar situations? Yes; but im not sure what exactly what it could be
Is there any other unusual activity/symptoms? Nope he seems fine
