I'd be testing it now and doing a water change. 25 litres is far too small for goldfish, you were probably missold it by the shop- it happens a lot. A 30% water change every other week isn't even enough in a tank that is big enough- I do near enough 50% of mine every week and it's a 60 litre with just a few tiny fish in there. With 2 goldfish in a 25 litre tank I'd be changing 30% a day until I could get them into a bigger tank (ASAP!)
How do you clean the filter sponge? It needs to be cleaned with water from the tank, never tap water because the chlorine will kill the good bacteria in the filter.
Finally, there's some info here on goldfish life expectancy and tank size recommendations. Please make sure you are sitting down with a very large gin before you read it. I don't want to be responsible for you keeling over in shock!
Sorry for all the bad news I hope you can get things sorted so that your fish gets better.
thanks, I will test water tomorrow I have a kit somewhere in the kitchen cupboard. We have 2 goldfish, tank probably too small, its a 25l, water change every week (30%), filter sponge cleaned. We've had the poorly fish for over a year, the other for about 8 months.
Oh dear, he doesn't sound in a good way If a fish has recurring problems like this it's usually a sign of poor water quality- have you got a test kit? Can you tell us the latest results?
If you don't have a test kit then some information about the tank and fish will narrow it down: what sort of fish and how many, the volume of the tank, the last time you cleaned the tank and filter and how long it has been set up.
It may not be the end but you'll have to move quickly to sort things out.
a while back his tail fin became what looked like ripped, treated it with finrot medicine, he seemed to recover although no tail fin grew back. A few weeks ago his tail began to disappear and white cotton wool stuff started to grow. continued to treat with the finrot medicine. No avail this time. Have treated with melafix for this week. It looks like its getting worse and poor fish is just sitting at the bottom of the tank looking like he is having trouble "breathing2 What can I do? Is this the end? I don't want him to suffer. Please help!