Hi and welcome.
You have realised the original tank was too small for goldfish. They are big waste producers too - like the apple snails.
Mixing meds and tonics is not a good idea either as some can react with others.
Goldfish do not need company to be happy. Ignore your heart
What sort of filtration do you have?
Here are my questions now:
1) I cleaned out all the gravel and ornaments and "disinfected" it with formalin - then rinsed it a few times and mixed in Prime, Stress Zyme and Stress Coat with the gravel to put the good bacteria back to fight whatever caused the ilnesses -
was this the right thing to do - or what else could I have done? I don't want the feisty one getting sick.
Are you sure the fish had finrot and not ammonia poisoning or other problems due to the poor water quality? Best thing to do in these cases is to improve the water quality. Did you ever test it for ammonia, nitrite or nitrates? This would have helped with a proper diagnosis. Forget adding products and do more water changes.
2) How long to wait before placing him in the new tank (much bigger at 54 l) with two other fish? When will I know it is okay to transfer him - what else can I do to make sure he is disease free?
54L is not big enough for 3 goldfish to grow. All you can do is get the water quality fixed then keep an eye on the fish for any sign of further trouble.
I found this on a goldfish specialist site...
Stocking level - what are the SURFACE dimensions (length x width) of your tank? As a guide, in a coldwater aquarium, you should allow:
60 square cm for each 1 cm of fish body length excluding the tail or 24 square inches for each 1 inch of fish body length excluding the tail.
For example, in a 60 x 30 cm tank you can keep a total of 30 cm (12 inches) of combined fish body length, such as three 10 cm-long fishes, or two 15 cm-long fishes.
There is no way round this! If you overstock, you will always be up against problems; remember, too, that small goldfish grow, so you need some spare capacity for growth of your existing stock. Do your sums, and, if you are overstocked, find temporary accommodation for your surplus fish until you either get a second or a larger tank.
Lessons learnt:
Apple snails are extraordinary creatures BUT not in a 21 l tank - leaves about 1 l of waste a week
They are great little poo machines aren't they? :roll: :slfg:
Water that is crystal clear is not necessarily healthy water
This is so true and inversely, cloudy water is not necessarily unhealthy either