my turtles cage

Question
My turtles cage has white foamy bubbles in it, he is a Texas red eared slider, and he has a red sore looking thing on his rear. I change the water about every 1-2 weeks and he has a average petco turtle cage,I'm not sure how many gallons...thats it

Answer
Well first of all I need to know the Temp. of the tank and does he have any tankmates?
If you have a pump for water circulation you should get a filter. The bubbles is probably from the circulation of the pump. Or maybe it's the turtle, I have 4 Texas Red-Eared Sliders and they make some bubbles sometimes. If you don't have a filter, you should get one. You can buy them @ a local pet shop/store such as Petsmart and Petco. If you only have 1 turtle, I advise you to get him a tank mate so he won't feel lonely.
-Sam
P.S. Don't feel shy to ask more questions!