cause of cloudy water

I have a 55 gal, lightly planted community tank with no CO2 or fertilizer
(yet), 2 HOT power filters together rated for over 100 gal tank filtration,
that has been up and running for about 6 months now. Just recently I've
begun to notice a slight cloudiness in the water, almost looks like very
fine bubbles. It does not seem to go away with water changes. I typically do
about a 25% water change every couple weeks w/Stress Coat for a
dechlorinator. I have just plain aquarium gravel for substrate. I have 10
tetras, some zebra danios, 4 corys, 2 yoyo loaches, a bushynose pleco, and 3
small angelfish. My levels are almost always the same pretty much according
to my test strips.

Nitrates: ~40ppm
Nitrites: 0ppm
GH: ~75ppm
KH: between 40 and 80ppm
pH: ~6.5-6.8

My tap water tests as follows:

Nitrates: ~10ppm
Nitrites: 0ppm
GH: 75ppm
KH: 80ppm
pH: 6.8

All my fish seem fine and happy. Any ideas? I've seen products for clearing
water. Is this something I should think about or is the cloudiness perfectly
natural/normal? One other thing to consider is that I have been adding
Melafix to the tank for the past few days to help with some minor fin rot on
my angels. I've only been adding the recommended dosage. Does anyone else
have experience with Melafix causing cloudy water?

Thanks,
Kristen