Hi VickiH,
I recently had the same problem with my tank that had been running for about 4 years without any problems, I thought it was algae or bacterial related due to gving the tank a good clean and changing the bulbs. After 3 weeks of dosing with algaefix and doing many large water changes I traced the problem to the Ceramic Noodles that I had added to the filter at the same time as I cleaned the tank. They were a cheap brand, about $5 per blue coloured box and came in there own mesh bag, if you have added these to your filter recently they may be the problem. My water was so cloudy, due to the noodles breaking down, that I couldn't even see the back of the tank and since I have removed the noodles my water is crystal clear.
I hope you resolve your problem soon.
Cheers,
Matt