I am not sure if it is mineral deposits or shell rot (I am a bit of a worrier)
I can't see how it could be shell rot as they have a very adequate set up - although we recently had to remove one of the turtles as he was being quite aggressive, nipping at the others feet - but I never noticed any open wounds or they would have been treated.
Their shells are hard, they are all still eating and active, there is no smell coming from them. The white patches seem to be hard - not pus-y or weeping.
They have a large cork basking area with a 10.0 UV and a basking lamp. Temps are around 31/32C and the water temp is about 21C.
I have included a picture - all views are welcome. If they need to be treated I want to get on top of it right away.
Thanks.
EDIT - Sorry I forgot to mention the only recent change to the set up has been a new external filter. Fluval 405.





