goldfish and plecostmus

Question
I have a 40 gallon tank with 6 different, rather large goldfish, I have added a plecostomus, a large one to the mix. the water gets really cloudy i as little as a week and am having to put fresh water weekly into it. I have a pump filter and a heater, i try not to feed it too much and still is really cloudy, also fish are gulping water at top of tank even though i have air pump for bubbles and a water fall filter system? can u please give me advise?

Answer
You need a much larger tank. Your problem is not the feeding at all.  40 gallons is not even enough room for a fully grown single goldfish...and you have six.

Goldfish require atleast 5 gallons per inch of them and they get a foot long. 12x5= 60.  That is at least 60 gallons for one goldfish. Sometimes a 55 gallon can work for one. Let's take that 55-60 gallons times those 6 goldfish. 330-360 gallon tank is what you need to comfortably accomodate all these fish.

5 gallons per inch, 12 inches, and 6 of them is a lot.  

A pleco will not keep your water from getting cloudy they just eat algae. I would definately invest in a bigger tank or get different tanks and seperate them. Goldfish need A LOT more room.