Start with the right tank size and location

A 20-gallon tank is the easiest size for a beginner because it holds water temperature more steadily than smaller tanks and gives fish room to swim. Smaller tanks swing between too hot and too cold, which stresses fish and makes them sick. A 10-gallon tank works if space is tight, but you will have fewer options for which fish you can keep.

Place your tank on a sturdy, level surface away from direct sunlight and heating vents. Sunlight causes algae blooms that turn the water green. Heat from vents and air conditioning units makes the water temperature unstable. A corner of a living room or bedroom, away from windows, works well. Make sure the surface can hold the weight—a 20-gallon tank filled with water, gravel, and decorations weighs about 225 pounds.

Do not put the tank on a dresser, bookshelf, or any furniture not designed to support aquariums. The weight will eventually cause the furniture to sag or collapse, and the tank will crack.

Key Takeaways

  • A 20-gallon tank is the best starting size because it maintains stable water temperature and gives you flexibility in which fish to keep.
  • You need a filter, heater, air pump, and thermometer before you add any fish—these run the tank, not the fish.
  • Gravel, plants, and decorations give fish places to hide and reduce stress, which keeps them healthy.
  • Let the tank run for one to two weeks before adding fish so bacteria can build up and break down fish waste.
  • Test the water weekly for ammonia and nitrite for the first month, then monthly after that.

Gather the equipment you need before filling the tank

You will need a filter, a heater, an air pump, and a thermometer. These are not optional—they do the work of keeping fish alive. A filter removes waste and uneaten food. A heater keeps the water at the right temperature (usually 76 to 78 degrees Fahrenheit for tropical fish). An air pump adds oxygen. A thermometer tells you if the heater is working.

Buy a filter rated for your tank size or larger. A filter rated for 20 gallons will work on a 20-gallon tank, but one rated for 30 gallons is better because it cleans faster. Hang-on filters clip to the back of the tank and are the cheapest option. Canister filters sit under the tank and cost more but clean very well.

For a heater, buy one rated for your tank size. A 100-watt heater works for a 20-gallon tank. An air pump is optional if your filter creates surface movement (you will see ripples on top of the water), but adding one does not hurt. A straightforward glass thermometer costs a few dollars and sticks to the inside of the tank.

Add substrate, plants, and decorations

Substrate is the material on the bottom of the tank—usually gravel or sand. Rinse gravel in a bucket under running water until the water runs clear. Unrinsed gravel clouds the water and makes it hard to see. Spread 2 to 3 inches of rinsed gravel on the bottom of the tank.

Add plants and decorations next. Live plants are better than plastic ones because they absorb fish waste and produce oxygen, but plastic plants work if you do not want to maintain live ones. Arrange plants and rocks so fish have places to hide. Fish that feel exposed get stressed and stop eating. A stressed fish is a sick fish.

Do not use decorations from outside—rocks from a river or yard may contain bacteria or chemicals that harm fish. Buy decorations from a pet store or use items you know are safe, like smooth stones from an aquarium supply store.

Fill the tank and run it for one to two weeks before adding fish

Fill the tank slowly with water from your tap. If your tap water is very hard or very soft, you may need to treat it, but most tap water is fine for fish. Place a plate or bowl on the gravel and pour water onto it so the water does not blast the gravel and plants around.

Turn on the filter, heater, and air pump. Check that water is flowing out of the filter and that the heater is warming the water. The thermometer should read 76 to 78 degrees within a few hours. If it does not, the heater may be broken—turn it off and replace it.

Let the tank run for one to two weeks without fish. During this time, beneficial bacteria grow on the gravel, plants, and filter. These bacteria break down ammonia (fish waste) into less harmful compounds. If you add fish before these bacteria are established, ammonia will build up and poison the fish. This is called cycling the tank.

Test the water before adding your first fish

Buy a water test kit from a pet store. The kit measures ammonia, nitrite, and nitrate. After one to two weeks of running, ammonia and nitrite should be zero or very close to zero. Nitrate will be higher—that is normal and expected.

If ammonia or nitrite are still high after two weeks, wait another week and test again. Do not rush this step. Fish cannot survive in water with ammonia or nitrite above 0.5 parts per million.

Some pet stores will test your water for free if you bring in a sample. Call ahead and ask. This saves you the cost of a test kit if you only plan to set up one tank.

Add fish slowly and watch them for the first week

Start with hardy fish that tolerate beginner mistakes. Goldfish, bettas, and tetras are good choices. Do not add more than three or four fish to a 20-gallon tank at first. More fish means more waste, which can spike ammonia and kill them.

When you bring fish home, float the bag they came in on the surface of the tank for 15 to 20 minutes. This lets the water in the bag slowly match the temperature of the tank. Then open the bag and let the fish swim out. Do not pour the bag water into the tank—it may carry disease from the pet store.

Watch the fish for the first week. They should eat when you feed them and swim around actively. If a fish hides all day or stops eating, it is stressed. Remove it and return it to the store, or move it to a separate tank if you have one. Do not add more fish until the first batch is healthy and eating well.

Feed fish once a day and change water weekly

Feed fish a small pinch of food once a day. They should finish it in two to three minutes. Uneaten food sinks to the bottom and rots, which raises ammonia. It is better to underfeed than overfeed.

Change 25 percent of the water every week. Siphon out water from the bottom of the tank where waste collects. Fill a bucket with tap water, let it sit for 24 hours so chlorine evaporates, then pour it back into the tank. If your tap water is treated with chloramine instead of chlorine, buy a dechlorinator from a pet store and add it to the bucket before pouring.

Test the water once a week for the first month, then once a month after that. Ammonia and nitrite should stay at zero. Nitrate will climb over time—when it reaches 40 parts per million, do a larger water change (50 percent instead of 25 percent) to bring it down.

Frequently Asked Questions

How long does it take to set up an aquarium?

The physical setup takes two to three hours. But you should let the tank run for one to two weeks before adding fish so bacteria can grow. Total time from buying the tank to having fish is three to four weeks.

Can I use tap water straight from the faucet?

Most tap water is fine for fish. If your water is treated with chlorine, let it sit in a bucket for 24 hours before adding it to the tank—chlorine evaporates on its own. If your water is treated with chloramine, buy a dechlorinator at a pet store and add it to the bucket.

What if my fish die after I add them?

Fish usually die because ammonia or nitrite is too high, the water is the wrong temperature, or the tank was not cycled long enough. Test the water and check the thermometer. If ammonia or nitrite are above zero, do a 50 percent water change and wait another week before adding more fish.

Do I need a heater if my house is warm?

Most tropical fish need water between 76 and 78 degrees. If your house stays that warm day and night, you may not need a heater. But a heater is cheap insurance—it keeps the water stable when the house cools down at night or the air conditioning kicks on.

How often should I clean the tank?

Do not do a full cleaning. Instead, siphon gravel once a week to remove waste, and change 25 percent of the water. Clean the filter once a month by rinsing it in old tank water (not tap water) so you do not kill the bacteria living on it. A full tank cleaning kills the bacteria and restarts the cycle.