Do you have a basking bulb set up for your turtle? Most turtles require an ambient air temperature in their tank to be minimum of 76oF with a max of about 80oF for basking.
The reason your water is not keeping warm is because you have an open top. That will disappate heat rapidly if your room is a cold sink. But open tops are required for ventilation in a turtle tank.
And I personally wouldn't put a water heater in a turtle tank. Turtles are notorious for being wreckless around glass, and many heaters, get broken by a vigorous turtle. Besides, turtles use their environment to thermoregulate, they need the water to be cool so that if they get too warm they can slip into it.
What type of turtle and why are you housing it with fish anyway? Most water turtles sold on the market today are carnivors that are going to eat your fish in the long run. While it is possible to do fish/turtle set ups as Soulfish describes (I too volunteer in a nature center), it usually requires more water and space than just a 30 breeder.
~~Colesea