I too have a tank and it goes cloudy almost immediately after changing the water.I found two products that clear the water over night.One is called "Water Clear" made by Jungle and the other is called "Crystal Clear" made by AP. Both can be found at Wal-Mart.
Usually a filter will fix it, but can take a day, also get some water ager and conditioner from the pet shop, and dont put any fish in the tank for 4 days, this gives time for the tank ecology to settle and chemicals like chlorine to disperse. Trick to a good tank is to let the water age for a week and add plants, then after a week add some fish. Best of luck with your fish, and every 4 weeks only change a quarter of the tank water, can siphon it out and run the siphon over the pebbles to suck out the fish leftover stuff.