Don't add chemicals unless you know what the problem is. Catching a sick fish and moving them can stress them even more, and using chemicals when they aren't needed can harm fish too. Most fish medications are poisonous to fish and other animals, but if you use the correct dose, you can kill the disease without killing the fish.
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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
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The fish is a small fantail goldfish and it has to compete with a couple of bigger common goldfish. This is going to make it hard for the little one to get food and remain healthy. You will need to target feed the small ones so they can get enough food to grow, otherwise they will waste away and die.
Goldfish need lots of plant matter in their diet and you could grow Duckweed and Ambulia in the tank. The fish will eat these plants and it will help their digestion and boost their immune system. You can also buy algae from health food shops or the Asian food isle in a supermarket.
You can also feed them on raw/ cooked prawn and other seafood. Get some frozen prawn and keep it in the freezer. Take one out and defrost it. Then remove the head, shell and gut (thin black tube in body) and throw these bits away. use a pair of scissors to cut the remaining prawn tail into small pieces and offer a few bits at a time. Feed until everyone has eaten their fill.
You can also buy marine mix (consists of prawn, fish & squid) or marine green (has prawn, fish, squid and spinach or algae) from pet shops. It is kept in the freezer and you pop a block out of the pack, defrost it and offer a bit at a time to the fish.
Other frozen foods can include brineshrimp, daphnia, msysis shrimp and bloodworms. All available form pet shops.
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You need a sponge over the intake of the powerhead so the fish don't get sucked onto it and killed.
There might be too much water movement in the tank and I would remove the powerhead or at least turn it off for most of the day to give the fish a calmer area to swim in. Whilst the common goldfish can deal with more current, the fantails (double tail goldfish) can't. The fantails have a shorter body and 2 tails, which makes swimming much more difficult, and if they have to content with a strong current and lack of food due to bigger faster moving fish, they won't do well.
