Step 1: Tank size
For fancy goldfish (fantails,pearlscale,celestial etc) you should have 80 litres (20 gallons) for the first one and 40 litres (10 gallons) for each additional.
Long bodied goldfish and celestials should be kept in a species only tank.
Step 2: Filteration
You should buy a filter capable of filtering the tank water 10 times an hour.
So buy the most powerful filter you can afford.
You will also need to cycle the aquarium
This can take up to 4 weeks.
Step 3: Feeding
I would feed a small goldfish the amount it can consume in 2 minutes every day
and a large goldfish a larder amount daily.
Choose a goldfish food containing low amounts of flours and wheats and higher amounts of vegetation and meat such as worms and shrimp.
Step 4: Maintenance
# Always use water dechlorinator. This removes the chlorine and chloramines. from the water.
# Add a little aquarium salt to help fight infection about 1 tsp per 20 litres (5 gallons).
# Wash out the filter cartridge in old tank water monthly. Do not rinse the cartridge so much that all the bacteria comes out of it, or you will have a re-cycle and your fish will die. Just rinse the cartridge a little so water flow is normal.
Step 5: Adding the goldfish
1. Float the bag the fish is in for 15 minutes.
2. Add 1 cup of your cycled aquarium water to the bag.
3. Wait 10 minutes.
4. Gently net the fish into the aquarium and discard the bag water.