Blackish layer on my Molly and DG

Hi All,

Yesterday I upgraded my 15g (60L) to 50g (200L) using all the established gravel, filter media etc from 15g that has been running for the past 3 yrs. Fish in tank
1 molly
1 balloon molly
1 DG
2 upside down catfish
1 common pleco (love this guy!)
4 zebra danios (added today)

Tested water this morning and all good
Nitrite 0
Ammonia 0
Nitrate 40
PH 7.2-7.4

Have to mention here that Nitrate in 60 L was pretty high for a week or more 80-120ppm (lost my API master test chart, using scanned image at the mo so difficult to be 100% sure of result)

Then today i noticed that two of my fishies have a strange blackish layer on them. The molly's fin also seems to have a black edge and the DG seems to have this blackish stuff around his mouth area. Both feeding well.

Added 4 zebra danios this afternoon (before i noticed the black stuff, otherwise wouldn't have done so) and things went downhill from there. First I noticed the black stuff, then i noticed that 2 of the danios have eyes missing. Placed them in qt tank for observation but they seem to be doing fine.
This evening I noticed that my molly (very pregnant at the mo) keeps dissappearing and hiding around the tank. Now I wouldn't think too much of it under normal circumstances since she is about to drop the fry pretty soon, but my balloon molly and the DG are doing the same thing and seem to krep to the bottom of the tank.

Are they sick and if so, what could it be and how do I treat it? Have had my aquariums for three yrs but haven't had to deal with a lot of diseases to date so no idea what to do

PS: pics attached are not great, I know, couldn't get them to stay still