Odd Aquarium

If water changes are not working I would add something to the tank called accu-clear or any other additive that say the clear aquarium water.

try a gravel vac? there could be a lot of debris or pooh and stuff in the that stays in the gravel and then more is released into the water after adding new water. or the water you're inserting may be cloudy. (which is when you should use anything that would chemically clear the water)

Is the cloudiness white, or greenish? (Take a clear glassful out of the tank and hold it against something white under good lighting if you're not sure).
White is generally bacterial - often settles on its own, it can be caused by the tank getting a little 'off balance' and sometimes takes time to settle. Best remedy is often to leave things alone (don't stop regular water changes, but try not to do anything major with the substrate or filter).
If it's green, that's algae, and another thing entirely. Free floating unicellular algae can be horrid to try to get rid of. One surefire fix is a blackout - do a big water change, then turn off the lights and completely cover the tank so no light gets in for 3-5 days. Uncover, and do another big water change. The algae should disappear - however, if the imbalance or excess nutrients/light that caused the problem isn't prevented in future, the algae will come back.
If the tank is just cloudy with debris, you can filter through fine floss and/or use a flocculant (chemical that makes little teeny particles stick together so they're large enough for the filter to collect), and that should take care of it.