It's ages since I posted here! anyways, I have noticed some black patches forming over the body of my common goldfish. He seems perfectly fine and is eating normally. Water parameters are fine.... what could this be?

ummm..i'm not a expert about goldfish..but..if a gold color goldfish jux turn black..that not a good sign...if a black turn to yellow or gold..that a good sign..U ever heard of ammonia burn...i think when a fish have ammonia burn..he will turn back...but..since u say ur water is good..then..i donno..what to say...hope ur goldfish turn black is not a ammonia burn..i hope is part of nature that he/she turn black..Wish best of luck...hope someone know more about fish keeping will give u a 100% good answer back to u..*Wink*

Currahman... I am moving this to the disease/diagnosis section and hopefully we can find you some answers there. In the meantime could you post the paramatars that's asked above in the red part of the disease section. thanks

Sounds like ammonia burns that are healing. We need you to answer all the questions in the box above. Thanks.

However, if the burns are healing don't just assume it is ok, testing that water is most important.

it cannot be ammonia... my water has 0mg of it... 0 nitrites, nitrates at 20... ph neutral. I use 2 emperor 400 bio wheel filters on a 55 gallon tank. Other inhabitants show no signs of this whatsoever... Fish seems, I as I said, perfectly healthy otherwise
I change the water like clockwork, a third of it, weekly

Have you tested the tap water? Please post those as well.
Since you have the tank fully cycled, if there is ammonia in the tap water it can hurt them, but the cycle get rid of the "evidence" before you see it on the tests.

Can you get a pic of the black marks? Also you have answered alot of the questions, but we really need then all to have a clearer picture of what is going on.