hey i wouldnt worry about the algae growing on his/her shell if if were you. i have a buddy who uses a basking light in his turtles tank and it really gets the algae growing fast im talking by the end of the first day after he cleans the tank it is very apparent. i only use a uvb strip light over my turtles tank and i use an underwater heater to keep their temperature right around eighty/eighty two degrees and no algae grows in their tank at all. now i have four common razorback musk turtles and i use an underwater heater coupled with a basking light and algae grows underneath the light. I have read a couple of different places that algae isnt necessarily a bad thing and if you have it growing in your tank it shows that you have a good set up with all the right balances. good luck with your turtle man. let me know how things go.......