water clairity

Question
hello, i have a 55-gallon tank with a 10in arowanna, 12in plaucastamis(?), 3in bala shark and another little 2in guy (chiclid?).  i have two aqua clear 200 filters on the tank along with a heater.  i have a Eheim canister on it's way to replace the aqua clears.  
my question is or rather my problem is  the water always looks cloudy and i change the water 20% every other day.  i also check the quality and the nitrates are always 40ppm or higher.  i've checked the tap water and it was 0ppm on nitrates and nitrites.  i will admit that i used to never do water changes, just fill up what's evaporated.  but i didn't know any better until my 12in oscar got hole in the head so bad that it killed him.  so what am i doing wrong now that is keeping the water cloudy and the nitrates up.  i'm not overfeeding and that's all i can think might be the problem.  thanks.

Answer
Hi, it may be a good idea to change at least 65% of the water in one shot and refill it back.It may also be wise to change both filter carchrages and make sure they are cleaned whell.Another thing to buy at any local pet store or even wall mart activated carbon rocks wich remove fish wast and clear up your water withing a few hours.Once you have done this please e-mail me and tell me what has happed, reson being i am making a survay on wich questions have been answerd correctlly and have actually been succesfull.THANKS!