Start with the right tank size and location

A 20-gallon tank is the smallest practical size for a beginner. Smaller tanks swing wildly in temperature and water chemistry, making fish sick. Larger tanks are more forgiving and easier to maintain. Place your tank on a sturdy, level stand away from direct sunlight, heating vents, and air conditioning units — these cause temperature swings that stress fish.

Before you buy anything, measure the space where the tank will sit. Account for the stand, the filter hanging on the back, and room to reach inside for cleaning. A 20-gallon tank is 24 inches long, 12 inches wide, and 16 inches tall. A 40-gallon breeder tank is 36 inches long and takes up noticeably more space. Write down your dimensions and bring them to the store.

Key Takeaways

  • A 20-gallon tank or larger gives you room for error and lets you keep more than one fish without constant maintenance.
  • You need a filter rated for your tank size, a heater set to 76–78 degrees Fahrenheit, and a thermometer to monitor temperature.
  • Gravel, plants, and decorations go in before you add water, and the tank must run for 5 to 7 days before you add fish.
  • Test the water with a kit that measures ammonia, nitrite, and nitrate before adding your first fish, and do partial water changes every week.
  • Common beginner mistakes include overstocking, overfeeding, and changing the water too often, all of which kill fish by spiking ammonia.

Gather the essential equipment

You need four things before the tank can run: a filter, a heater, a thermometer, and a light. The filter must be rated for your tank size or larger — a filter rated for 20 gallons is the minimum for a 20-gallon tank. A hang-on-back filter (the most common type) clips to the rim and pulls water through a cartridge. Replace the cartridge every month, or rinse it in old tank water to keep the bacteria that break down fish waste.

The heater should be adjustable and rated for your tank size. A 50-watt heater works for a 10-gallon tank; a 100-watt heater works for a 20-gallon. Set it to 76–78 degrees Fahrenheit for most tropical fish. A straightforward glass thermometer that sticks to the inside of the tank costs a few dollars and tells you if the heater is working. The light is optional for fish health but makes the tank visible and helps plants grow if you add them.

You will also need a water test kit that measures ammonia, nitrite, and nitrate. A liquid test kit (not the paper strips) costs $20 to $30 and lasts for hundreds of tests. You need this before you add fish.

Add substrate, plants, and decorations

Rinse your gravel under running water in a colander until the water runs clear — this removes dust that clouds the tank. Spread 2 to 3 inches of gravel across the bottom. Gravel gives bacteria a place to live and helps break down waste. Dark gravel makes fish colors pop; light gravel shows debris more easily.

Add decorations, rocks, and plants before you fill the tank. Live plants absorb some of the ammonia that fish produce, but they need light and time to grow. Plastic plants do not help with water chemistry but look good and give fish places to hide. Arrange decorations so fish have open swimming space in the middle and hiding spots along the edges. Avoid sharp edges that can tear fins.

Fill the tank and run the filter for one week

Fill the tank slowly with room-temperature water. Pour water onto a plate or into a cup to avoid disturbing the gravel. Turn on the filter and heater. The water will be cloudy for a day or two — this is normal and clears on its own as the filter catches particles. Do not add fish yet.

For the next 5 to 7 days, the filter will grow bacteria that eat ammonia and nitrite. This is called cycling. You cannot see it happening, but it is essential. During this time, check that the heater is holding temperature and the filter is running quietly. If the filter is loud or leaking, fix it before you add fish.

Test the water before adding fish

After 5 to 7 days, use your water test kit to measure ammonia, nitrite, and nitrate. Ammonia and nitrite should both read zero. Nitrate will be low but not zero — this is normal. If ammonia or nitrite are above zero, wait another few days and test again. Do not add fish until both are zero.

Once the water is safe, you can add your first fish. Start with hardy species that tolerate beginner mistakes: tetras, danios, corydoras catfish, or guppies. Add only 3 to 5 fish to a 20-gallon tank. More fish means more waste, which spikes ammonia and kills them. Wait two weeks before adding more fish — this gives the bacteria time to handle the extra load.

Feed fish once a day and change water weekly

Feed fish a pinch of food once a day, not multiple times. Fish have small stomachs and do not need much. Uneaten food sinks to the bottom and rots, spiking ammonia. If you see food on the gravel after five minutes, you fed too much. Remove it with a small net.

Every week, remove 25 percent of the water with a siphon or a bucket and replace it with fresh, room-temperature water. This removes waste that the filter cannot catch and keeps nitrate from building up. Do not change all the water at once — this kills the bacteria and crashes the tank. Never use tap water treated with chlorine without letting it sit for 24 hours first, or use a dechlorinator product.

Watch for common problems and fix them early

Fish gasping at the surface means the water does not have enough oxygen. This happens when the tank is overstocked or the filter is clogged. Clean the filter cartridge and reduce the number of fish. Cloudy water that does not clear in a day means the filter is overloaded — do a 25 percent water change and feed less.

Dead fish should be removed when ready. If a fish dies, do not add a replacement right away. Wait a week and test the water. If ammonia or nitrite spike, the tank is overstocked. If they stay at zero, the tank is stable and you can add one new fish. Ich (white spots on fish) and fin rot (torn, ragged fins) are signs of poor water quality or stress. Do a 50 percent water change, clean the filter, and reduce feeding for a few days.

Frequently Asked Questions

How long does it take to set up a tank before I can add fish?

Five to seven days minimum. The filter needs time to grow bacteria that break down ammonia. If you add fish before this happens, ammonia spikes and kills them. You can speed this up slightly by adding a bacterial starter product, but you still need to test the water before adding fish.

Can I use tap water straight from the faucet?

Tap water contains chlorine that kills fish. Let it sit in an open bucket for 24 hours so the chlorine evaporates, or add a dechlorinator product (a few drops per gallon). Some cities use chloramine instead of chlorine — this does not evaporate, so you must use dechlorinator. Check your city's water report online if you are unsure.

What if I overfeed and the water gets cloudy?

Stop feeding for one day. Do a 25 percent water change and clean the filter cartridge by rinsing it in old tank water (not tap water, which kills bacteria). Feed less going forward — a pinch once a day is enough. If the water stays cloudy after two days, the tank may be overstocked.

How often should I clean the gravel?

Use a gravel vacuum during your weekly water change. Push the tube into the gravel to suck up waste without removing too much water. Clean only the top layer — you want to keep the bacteria living deeper in the gravel. A full gravel cleaning is not necessary and can crash the tank.

Can I move the tank once it is set up?

Moving the tank disturbs the gravel and can kill bacteria. If you must move it, drain most of the water into buckets, move the tank, and refill it slowly. Do not move the filter or heater — keep them running. Test the water the next day to make sure ammonia and nitrite are still zero.