If your goldfish is the single tail slim bodied type (common goldfish), then you are going to need a MUCH larger tank. A single tail common goldfish requires a bare minimum 55 gallon tank with a very strong filter. Fancy goldfish (those with double tails) require a bare minimum 10 gallons of water per fish.

Goldfish will live well in excess of 15 years in optimal conditions. Unfortunately, your goldfish will not live long in a 2 gallon tank. The goldfish's internal organs will continue to grow, while the outside of the goldfish will be unable to grow due to the extremely tiny container it lives in. This is incredibly painful and will result in the premature death of your fish.

Goldfish are extremely messy and produce a massive amount of waste. This waste turns into ammonia which is lethal to fish. In a properly sized aquarium, you would be able to cycle the tank which would eliminate the ammonia and nitrite. In your tiny 2 gallon container, you will never be able to establish this biologic filter. You MUST perform a 25% water change EVERY DAY to keep the ammonia levels in check. Use tap water treated with conditioner to neutralize chlorine and chloramine. You should also have an ammonia test kit.

I don't mean to be rude, but if you really want your goldfish to have a happy enriched goldfish life, then get a 55 gallon heavily filtered tank immediately or find someone with a fish pond willing to take your fish. Single tail goldfish are best suited for ponds.

The fin rot is caused by the extremely poor water quality the goldfish is living in. Basically, the fish is living in a toilet bowl filled with excrement and urine. This produces ammonia and depletes oxygen. This is why your fish was "flashing". The fish couldn't breathe because its waste depleted all the oxygen in the tank. This is why you need a MUCH bigger tank with heavy duty filter. Your fish may also have parasites. Ich is very common in tanks with extremely poor water quality.

he would be acting oddly in a bowl due to toxic ammonia i would imagine. your common goldfish has the potential to live over 20 years and grow to around 14" long.

if you want him to live a long and happy life, i would suggest you start to look into buying him a minimum of a 20 US gallon tank. once he gets to around 6" he's really going to do best in a pond.

goldfish are exceptionally messy fish and really need heavy filtration and lots of oxygenation (caused by surface water movement and air bubblers). the fin rot is likely to be caused by water quality as they are so messy they produce a lot of toxic ammonia. although the water changes are good for the water parameters, they're not so good for the fish, the constant changing environment can be quite stressful for them.

i would suggest you invest in a 20 gallon tank if possible, and cycle it before your fish moves in, see the links below. you'll be able to jump start the cycle with the gravel from your current tank, as well as store bought cycling products.

Instead of using drops use Maracyn, just incase the fin and tail rot leads to fungus. Th temature should be around 60 degrees, otherwise it can be unhealthy.