Black fins

Question
Hi Christy

Hoping you can help me. I have an orange fantail that I bought about a year and a half ago and he lives in a 70 litre tank. Just recently he's developed black markings on the edges of his dorsal fin and anal fins. Initially I noticed the marking on his dorsal fin only so the colouring seems to be 'spreading' to other fins. The fins themselves don't look frayed or in bad condition and he seems to be completely happy. Just wanted to know whether it's something to worry about. I have other fantails in the tank and was worrying about this thing spreading to them as well.
I do water changes once a week (1/3 of water is changed) but haven't tested any water readings such as ammonia/nitrate etc. I've had the tank for about 4 months.

Would be grateful for any advice!

Many thanks.

Kim

Answer
Hi Kim
I'm not too familiar with the fancier types of goldfish.  I have seen photos of some with black fins, and it is common for goldfish to change colors.  However, since you mention it's on the edges of the fins, and you mention there's others in the tank, I would get a test kit and test the ammonia and nitrite levels.  They both should be reading 0 ppm, and nitrates under 20 ppm.  It's recommended to have at least 57 litres/15 gallons for one goldfish, larger tanks when they get bigger/older.   So, it could be the start of fin rot you're seeing, especially if those water parameters are off.  Get a test kit and test those, and you'll probably need to consider a larger tank as well.  If the water quality is off, I would increase the water changes during the week until you can get a larger tank for them.  If the water quality is good, I would just keep an eye on the fins for any signs of shredding or tearing, and it may just be normal coloration for him starting to show up.

Hope that helps and good luck!

Christy