my goldfish gary has developed some abnormally black patches on the top of his back fin. These are on bothe sides. I first thought it might be fin rot but now i don't think so. THey are about 1 cm long and 0.5 cm high. My goldfish doesn't seem to be disturbed by these as he is happy and active and eats plenty of food. Any suggestions on what it could be? I have another goldfish in the same large tank...although it is a comet goldfish it has none of the symptons of my other fish. Please help as i am really worried about my fish. thanks

My tank is about 30 gallons and has two goldfish currently living in it. It has proper filtration but no snails or anything. I feed them on tetrafin flakes and sticks. The water is changed about 1 a week and 25 -50% is taken out and exchanged

The black patch seems to get bigger everyday and seems worrying
Could it be fin rot?
is there any other things like a flat....fairly large black patch that is with a disease.

I'd be really grateful of any replies
thnx

Hi Gary

Black patches often indicate healing ammonia burns (finrot would show itself as frayed fins with red streaks and white edges, so it definitely isn't that). Can you test the ammonia, nitrite, nitrate, pH and post the results here? Is there any reason why the tank's cycle might have stumbled recently? When you change the water, what dechlorinator do you use? Have these black patches appeared very gradually or quite suddenly?

Sorry for all the questions!