Update On New Tank And Some Question Re Water Paramaters

Hi all, hope you and your fish are all well.
Thought I'd give an update as not posted for a wee bit and as usual I've got some questions I'm hoping you can help with. 
I've now got 15 Harlies and 5 Skunk Corys and already find them fascinating! They seem to be settling in OK, oh and a snail has appeared, I assume more will follow!!
Question time: -
The water is still cloudy, and when a say this I mean when you look close you can see "stuff" floating through the water, is this what everyone is referring to when they say cloudy, or it is something else? I am right to assume it will eventually clear?
The brown stuff still appears on the leaves of the plants, but doesn't seem quite as bad, again assume it'll go in time.
Been doing weekly water changes and once did two in one week as had a minor ammonia panic. Today water results are
PH 7.4
Ammonia 0
Nitrite 0
Nitrate 0
Is it weird that there is no Nitrate and do I need to worry?
KH GH questions....I've got an API test kit for this, but don't really know what it's telling me...the GH turns at 6 drops so does this equate to 107.4ppm? Which is higher than what the water company said for my water, but suppose this testing is more "current" that what they had on their website? So when looking at fish it's this figure in relation to the hardness, yes?
The KH turns at 3 drops so does this equate to 3dKH and if so what does that actually mean, I think it's to do with how stable my pH will be but don't know if that's good or bad and whether I need to do something about it.
Not cleaned the filter yet, hoping that's OK as I've only had fish since the 28th Jan and still just have 20 fish?
And finally, what to do with the snail/s, do I leave it, live and let live, or get rid, no idea what kind it is, assume is just come in from the plants? 

As always your advice is appreciated 

Hiya, thanks for the advice. The filter has got what it came with, I've just checked the manual and it says, Bio-Foam, BIOMAX, fine pads and carbon, so I'll try the wool, any view on what I should take out if putting wool in?
I'll check the Nitrate again, but I've been shaking the bottle, and the tap water is zero for nitrate too. 

I'll leave the snail for a bit, it's just a wee thing