Is My Tank Over-Stocked + What Kind Of Snail Might This Be? + Molly Ac

Hello,
 
I recently got back into fish keeping. I've had three goldfish that I won from the county fair, but they were getting too large and dirty for my 20 gal tank. I decided to get rid of the goldies & start a community tank again. There is a lot I do not remember so I have a few general questions here Well for one, is my tank over-stocked? It is 20 gallons with a 20 gallon filter. I currently have 3 mollies- large male sail fin, female dalmation sail-fin, and a smaller, female regular creamsicle molly. I also have 5 danios and 3 emerald green cory catfish. Plus a large snail. 
 
Not sure if these guys have any specific name (just curious) does anyone know what type of snail he might be? I found him in the pond in my backyard. I live in North Florida. 
 
And for my last question! My big black male molly was very active at the pet store, but since I brought him home yesterday afternoon he has been quite sluggish and just sitting at the bottom or behind the filter. I tested the water and all the levels were fine. I'm not sure what is wrong with him. Will it just take a while for him to settle in?
 
Here are a few pics of the tank and snail (not sure if you can tell from the pics). 
 
ps, the ugly colorful things at the bottom of the tank is multi colored gravel. Going to cover them up with black soon. They make the tank look too cluttered to me.

hi the snail u have looks like a tropical apple snail.do u have a heater in ur tank as mollies are usually tropical fish not cold water.

I have a heater, although the tank is at about 78 degrees right now without it. 

What are your exact water parameters?  
 
pH
NH3
NO2
NO3
kH
gH

And what are you using to do these tests? Paper strips or liquid-drop tests?

AH. Sorry, I forgot about this thread a bit until I just checked out my tank and my male molly does not look good at all. His fins are all clamped to his body and he was white pieces of skin or maybe something white growing in little patches all over his body. Is this ich? I do have medicine for it, but I'm not sure he will make it. BTW I was using the test strips...I will do another test in a minute as I don't remember all the levels..

Well, I treated the water for ick last night. Jamarcus the molly looks remarkably better. His fins are out and he is more active. Still has the white fluffy stuff on him though. Unfortunately my apple snail decided to crawl out of the tank and I found him on the floor

The ich meds probably didn't agree with it... possibly some copper in the med.  Snails can be sensitive to certain meds.