Cloudy Water in 55 gal Tank

[jamesmac]

I have four goldfish about 9 to 10 inches in a 55 gal tank. They've been in that tank for four years and suddenly the water is extremely cloudy. The ammonia levels are normal and the water is changed 15 gals at least once a week.

Any ideas? Could it been an algae or calcium?

Let me know if ya'll have any ideas on how to clear this water up.

James