Tutle Tank Water

Question
I have a red-eared slider (a baby boy), with a shell diameter about 6 in. in length and 4 1/2 in. in width.  He currently lives in a w=12 in., h=17 in., l=24 in. tank which I believe is 20 or 25 gallons.  I fill the water up to 12 in. deep leaving 5 in. of head room.  Overhead is the appropriate UV light for reptiles (specifically turtles).  I use med. and lg. size river stones for him to dig in and have an artificial piece of wood on the bottom and a floating platform.  The filtration is a Rena xP canister filter for a 45 gallon tank that I purchased about 2-3 months ago, this is when the problem began.  

The problem is that the water develops a milky appearance in only a week, which has not been a problem for the past three years that I have had him.  There are sponges, sacks of carbon and zeolite crystals, and bio-stars.  When I put the new water in I balance it to pH 7.0, get rid of chlorine, etc.  I also use an all natural, bacterial pond clarifier by PondCare called Ecofix that is suppose to keep water clean and clear, as well as break down dead algae and increase oxygen, and Stress Zyme for a biological filtration booster (which I bought after the Ecofix was not helping much).  

I am currently thinking of buying something that releases air bubbles hoping that the milky cloudiness in the water is trapped gases that could be more easily released with the aid of the air bubbles (plus, it looks better aesthetically).

Is that going to help the water, or should I do something else (take into account that I am a college student and currently cannot buy new and bigger tank and/or filter)?

Answer
Sorry for the delay in answering- I did not know about this section until today!

The tank is too small- for a 6" long turtle it really should be about 60 gallons. Since the filter should be about 3 times the size of the tank, the filter is too small as well.

Since you won't like those answers, try this:
- stop dumping so many chemicals in the water. They are costing you money and overworking the filter. Also, stop worrying about the chlorine so much. Most of it will evaporate out in 24 hours and it does not bother turtles a lot anyway.
- use a simple siphon vac to remove about 1/4th of the water and clean the river rock
- consider getting rid of the river rock (it is holding a TON of decaying debris!)