danio with very thick layer of slime

[defiance]

found it this morning near the bottom of the tank
its very lethargic and has a very think layer of slime on its body starting just behind its head ive pulled it from the tank but now im unsure what is wrong please help
water parameters are all ok and no other fish seem to be effected

[StuG]

Slime Disease? (images)

Aquarium Slime Disease

Just a guess. So hopefully others will chime in. It's also known as Costia, and is generally caused by other factors.

What size tank?
How many fish?
What, exactly, are the parameters?
How long have you had the tank?
These answers will help others help you.

[defiance]

Thanks for that link I could not find anything.
It's a 20 gallon long its been established for years

stocking.
5zebra danios
5glowlight danios
7 wmc minnows
1 hillstream loach
the danio in question is at least 18 months old. It seems more active now and the slime layer seems to have reduced a little
Parameters are
Nitrite 0
Nitrate .25
Ph 7.6

I'm out of the ammonia test liquid at the moment

Can't understand why it would be stressed though no new changes everyone else in the tank looks healthy
I have done a 30% pwc and will do smaller pwc for the next few days
I can live with loosing the one fish I just don't want this to destroy my tank.