Water Temp, Other animals in enclosure

Question
I have a custom 15G aquarium a friend loaned me for my baby R.E.S.  I have a floating "Turtle Dock," but no heat lamp.  The aquarium has a hood with a built-in light, which I turn on during the daytime.  I live in California, and it being summer here the outside temperatures reach the 90's often, and the water temp gets to be 84-86 degrees.  is it harmful for my turtle to be in water that warm, and how can I cool the tank?  

I am about to upgrade to a 20G(long) tank that says it is built specifically for turtles, and i was wondering if there were any other specific species that could be kept in the same tank.  A friend had recommended a cleaner fish, to be decorative and somewhat productive.  Any ideas, advice, etc?

Answer
1. Tank size for a RES should offer a minimum of 10 gallons of water per inch of turtle. A 15 gallon tank is a bit tight and does not allow much growth. Even a 20 gal. is a bit snug- the turtle should hit about 6" in a couple years and will need 60 gallons of water space at that point. Most keepers use tubs or kiddie pools at that stage.

2. Water temps- as long as you are around 85 during the day you should be OK. However- the basking site should be closer to 90, and some cooler and shadier spots should be available.

3. Fish are a mixed blessing- good at keeping things clean, but turtles nibble on fish and harrass them. Baby RES tend to get nibbled on by bigger fish.

A good site for RES is http://www.redearslider.com