Pet stores have aquarium starter kits. They should include everything you need.
Get the following, either in a kit or out:
- A 10- or 15-gallon tank
- A top for the tank with a light (comes with starter kits)
- Some gravel (come with some start kits but not all)
- A pump/filter (comes with starter kits) - preferably Aquaclear brand, very simple and hardy
- A water heater - preferably the ones that look like test tubes. Cheap and simple.
- A fish net
- Some plastic plants and a few rocks. Buy these at the pet store.
- Fish flakes
- A little thermometer; you can get some that stick to the tank
- Two 5- gallon buckets
Don’t buy any fish. Yet.
Find a sturdy place for your tank - a 15-gallon tank weighs 150 pounds - and set it there. You need access to an electrical socket. Put the gravel in the tank and fill it with water.
Set up the pump. It should come with a few filters. This is pretty easy; it comes in about five plastic parts. The filters should not have to be changed every week; every month or two is more like it. Set up the heater, which just clips to the top of the tank and hangs into the water. Safety tip: the cords from the pump and heater should either run UPWARDS to the power supply, or should run down, then go back up to the power supply.
Set up your plastic plants and your rocks in attractive styles and let the pump run for a few days. Try to arrange your rocks and such so that there are places for fish to hide. Fish like having safe places. Fiddle around with the heater until the water is stuck around 78 degrees.
After a few days, go get some fish. Small freshwater fish. Try to find a store that is actually a tropical fish dealer, not a generic pet store. Ask the salesman for 4 to 6 fish that are hardy and have pleasant dispositions. Do not get one fish of 6 different breeds; get 4 fish of the same kind, or 2 of one kind and 4 of another. Fish like being around the same kind of fish. Tell him you are starting a new fish tank; he will know which fish are best for new tanks. He will give you some fish. Make sure you have fish food.
You will be given your fish in plastic bags of water. Go home and place the bags in the tank and leave them for 15-20 minutes. Then cut a hole in the bags, let in some of the water, and leave them for 10-15 more minutes. Then let the fish out into the tank. Turn the light off for a little while; they find it less frightening.
Feed them twice a day. You should feed them enough that they will eat it all in two minutes. For 4-6 fish that’s VERY little food.
A 15-gallon tank might handle up to 8-10 very small fish. If you get that many, you may want to get an air pump with an airstone. Ask at the fish store. It’s easy to set up.
Every week have your kid pour some water into one of your 5-gallon buckets. After the water has sat for 3 days take a bucketfull of water OUT of the tank, get rid of it, and pour in the new water you’ve had sitting around for 3 days. (The purpose of the wait time is to get rid of chlorine.) Do this once every week or so. Make it a routine.
Your fish should thrive (e.g. live longer than six months, maybe.) If the tank gets dirty you may be feeding them too much. Don’t be afraid to go a day without feeding them just to let the tank catch up. Hungry fish are happy fish. Get a little brush for the sides of the tank. From this point on, just add more stuff as the need dictates and the tropical fish store people tell you you need. Get your kid to do the hard stuff.