The Plan: To get two small goldfish and see them thrive and mature, living a normal life span of 5 to 10 (or more!) years. These instructions will introduce the basics; use it as a starting point, but make sure you also read and learn about the unique goldfish.
Goldfish need a lot of space for a few reasons. They grow quickly. They are messy. They like steady, high quality water conditions. For two fish a twenty gallon tank will be big enough for several years. You can start with a ten gallon tank, but you may need a bigger tank within a year!
Get a power filter that hangs on the outside of the tank. Ones with sponge elements that are easy to clean are good. Biowheels seem to work. Flow control is also good; goldfish don't like to swim against a big current all the time.
At most what you should need are dechlorinating products. If your water is treated by chloramination (your water authority can tell you), ask your pet shop to help you get the correct products.
A syphoning vacuum for cleaning the tank and changing water. Two buckets that you can comfortably lift full of water; one for removing old water and one for aging fresh water. An algae scrubber for the tank glass. An in-tank thermometer.
Gravel: a tank with no gravel is easier to keep clean. If you do put in gravel, make it only a thin layer, say one quarter inch deep. Both you and the goldfish will find this easier to clean. Plants: the fish will eat live plants, but that's only bad for the plants. Light: a tank hood may be needed ... definitely if you have plants.
You don't need a heater. More likely, you'll need a fan to blow on the water surface in the summer, to keep the tank cool. An exception is if the temperature of your house varies a lot during the day ... then you might want a heater to keep the temperature from falling too far when the house cools down.
The tank must be on a strong level surface that can support the weight (ten gallons of water alone weighs 85 pounds). Do not put the tank where it will receive direct sunlight. Try to put the tank in a spot where the temperature is fairly steady during the day. Remember that you will spill water around the tank inevitably ... never put the tank on an appliance, antique, etc.
Fill the tank with water. If the water has chlorine in it, and you have not dechlorinated it chemically, wait three days before doing anything more..
This is really important. Fish produce ammonia as a waste product (goldfish produce a lot of ammonia!). It's very toxic to goldfish. There are two kinds of good bacteria, one that turns ammonia into nitrite (also toxic!) and another that turns nitrite into nitrate (relatively harmless). You need to give the tank time to develop these good bacteria on the surfaces of the tank and filter.
The traditional way is to initially introduce just one fish. For the first two months change fifty percent of the water every other day. And take water samples to the pet shop every two weeks to be tested. When the water tests show zero ammonia and nitrites, you can add the second fish and cut back on the water changes.
If you want to spend money rather than time, investigate using bacterial starter products like "Fritz Zyme #7". Or even better, get an already started biofilter from someone.
Select healthy, active fish from a tank of healthy fish. If there are any sick fish in the tank, it's a good bet all may become sick. Never buy fish that float at the surface whenever they stop swimming.
There are hundreds of varieties of goldfish. Pick what you find attractive. It is best not to mix wild-type goldfish and slower, heavy bodied fancy goldfish. The sleeker wild-type will get all the food in a mixed group.
Feed the goldfish as much as they can finish in two minutes, twice a day. Feed sinking food and, if it is dry food, soak it in water for a few minutes to moisten it before feeding. Supplement their diet with peas that have been popped out of their skins and blanched leaf greens.
Age new water for at least two days, so chlorine and other gases in solution are released, and so that the temperature is the same as that in the tank. If you need to treat for chloramines, follow the instructions you get carefully. In general, goldfish can adapt to almost any tap water hardness or pH, as long as changes are made slowly, over a few days. Your life will be easier if you just get your fish used to your normal conditions.
Use the syphon to clean the poop out of the tank once or twice a week. If you have gravel, make sure you use a syphon with a gravel vacuuming attachment and follow the instructions. If you have algae, clean it off the front glass but leave it elsewhere (the fish will nibble on it). Also use the syphon for water changing; change twenty five percent of the water each week. Finally, clean out the filter element every couple of weeks, rinsing it in used tank water (tap water could kill the good bacteria!).
Usually, doing all these things at the same time is most efficient.
Not very critical. A water temperature of 68 degrees Farenheit is considered ideal. Above 80 degrees some fish may suffer and even die. Below 50 degrees the fish will become lethargic and should not be fed.
If you find you enjoy your fish, consider consorting with others that feel the same! Participate in internet discussion groups, join the Goldfish Society of America, subscribe to aquarium periodicals, ... learning about goldfish and meeting new people can be an additional level of fun above and beyond keeping our finny little friends.