One of our mollies lost it's tail.

I went to do a 25%
water change over this past weekend and thats when we noticed it. He seems to be moving around fine, but he is staying in the top corner of the tank until I feed them. Once he eats, he goes back to the corner. He is also sticking his mouth out of the water and breathing I guess. He eats good and gets around good when he leaves the corner.
The water is CLOUDY! It has been this way since around the 3rd of Aug. I am just not sure how often I can do water changes, and what %, I can do to help clean it up. My filter media is good, but I did have to turn my filter down because we lost a
neon tetra as he got stuck on the inlet. I had turned it down to the lowest setting. Last night I turned it back to about medium. I am wondering if that will help with the cloudyness.
I checked my readings last night and I have 0 amm, 0 nitri and 20
nitrate. Thats to much isnt it? I am thinking another 50% water change tonight to get that back down. Is that ok?
Whats the best way to get rid of snails

Oh man, I am flooded with them.
Please check my tank info. How many more fish would you all recommend?
Lastly, any good battery powered gravel vacs out there?