Hello I'm new and one of my sons fish is I'll so I'm looking for advice. I have 2 Ryunkins in a 30l ( I think) tank, I have a Marina filter. I had new plants about a month ago and tank has been set up for app 6 months. The fish seems to have gone red under his/ her skin generally on one aide of it's head but a bit on top too. The other fish much bigger seems to be pushing at ill- fish so could be injury rather than illness. I'll try and post a pic , hoping someone knows what u can do(
Unfortunately the symptoms the fish is showing are typical for poor water quality as a result of being in a tank that's too small.
Fancy Goldfish should grow to be very large fish, around 8 inches long, and they produce a large amount of waste for their size because of their diet and digestion. For those reasons they need very large tanks to thrive and in small amounts of water they become stunted and usually suffer poisoning from their own waste as the water volume and filtration in small tanks just can't cope.
Do you have a test kit? If so, please test the tank and post your results for ammonia, nitrite, nitrate and pH. If not, yo need to get one asap. This is the one most of us use. It's cheaper on eBay than in the shops. Any liquid based test kit will be fine, but don't get paper test strips as they aren't very accurate.
Your fish will need a much bigger tank. For a long term home they need around 180 litres just for the two of them. This article explains about tank sizes.
To help them right away, start changing around 25% of the water in the tank every day for clean water, warmed to the same temperature as the tank and treated with dechlorinator.
when you have some test results we can advise further.