Start with the tank size and location you can actually maintain
The first decision is not what fish you want—it is what size tank fits your space and your willingness to do water changes. Smaller tanks (under 20 gallons) swing in temperature and chemistry faster than larger ones, which means they need more frequent maintenance. A 20-gallon tank or larger is more forgiving for a beginner. Pick a spot away from direct sunlight, heating vents, and air conditioning, because temperature swings stress fish and encourage algae blooms.
Once you have chosen the location, measure the space and write down the dimensions. This matters because you will need to know the exact tank size when you buy a filter, heater, and substrate. A standard 20-gallon tank is 24 inches long, 12 inches wide, and 16 inches tall, but tanks vary. Do not guess.
Key Takeaways
- A 20-gallon tank or larger is easier to maintain than smaller tanks because the water chemistry stays more stable.
- You must run the filter and heater for at least 24 hours before adding any fish, so the water temperature and flow settle.
- Substrate, rocks, and plants should go in before you fill the tank completely, so you can arrange them without draining water.
- The nitrogen cycle takes 4 to 6 weeks to establish, during which ammonia and nitrite levels drop naturally—test the water before adding more than a few fish.
- A partial water change of 25 to 30 percent every one to two weeks keeps nitrate levels down and prevents algae buildup.
Gather the equipment you need before filling the tank
You will need a filter, heater, thermometer, and substrate (gravel or sand). The filter size depends on your tank volume—most filters are rated by gallons per hour (GPH). A filter rated for 100 GPH works well for a 20-gallon tank. The heater wattage depends on tank size and room temperature; a 50-watt heater works for a 20-gallon tank in a room that stays above 65 degrees Fahrenheit.
Substrate comes in two main types: gravel and sand. Gravel is easier to clean during water changes because debris sits on top. Sand looks more natural and is better for bottom-feeding fish, but it compacts and traps waste underneath. Choose based on the fish you plan to keep. You will also need a bucket (dedicated to the tank, never used for cleaning chemicals), a siphon or gravel vacuum for water changes, and a water test kit that measures ammonia, nitrite, nitrate, and pH.
Optional but helpful: a timer for the light (so fish get consistent day and night), plants (live or plastic), rocks or driftwood for hiding spots, and an air pump if you want extra water movement. Do not buy fish yet.
Rinse the substrate and arrange the tank before adding water
Rinse the substrate in small batches with tap water until the water runs clear. This removes dust and prevents the tank from staying cloudy for days. Place the rinsed substrate in the empty tank to a depth of 1 to 2 inches. If you are using rocks, driftwood, or plants, arrange them now while the tank is empty and you can see what you are doing.
Position the heater and filter intake in the back or corner so they are less visible. Leave space for the filter outlet so water can flow across the tank. If you are using live plants, bury the roots in the substrate and leave the leaves free. Plastic plants can go anywhere. Once the layout looks right, you are ready to fill.
Fill the tank and run the equipment for 24 hours before adding fish
Fill the tank slowly with tap water. If your tap water is very hard or has high chlorine, you may need a water conditioner—your local aquarium store can test your tap water and tell you. Most tap water is fine without treatment. Place a plate or bowl on the substrate and pour water onto it so the current does not scatter your arrangement.
Once the tank is full, turn on the filter and heater. Check that water is flowing through the filter outlet and that the heater is warming the water. The thermometer should rise toward your target temperature (72 to 78 degrees Fahrenheit for most tropical fish) over the next few hours. Let everything run for at least 24 hours. The water will probably look cloudy—this is normal and clears on its own as the filter traps particles.
Test the water and understand the nitrogen cycle before stocking fish
After 24 hours, test the water with your test kit. You are looking for ammonia, nitrite, and nitrate levels. At this stage, ammonia and nitrite will probably be zero, and nitrate will be zero or very low. This is normal. What matters is that the filter is running and the heater is holding temperature steady.
The nitrogen cycle is the process that makes a tank safe for fish. When fish waste and uneaten food break down, they release ammonia, which is toxic. Beneficial bacteria colonize the filter and substrate and convert ammonia to nitrite (also toxic), then to nitrate (much less toxic). This cycle takes 4 to 6 weeks to fully establish. During this time, ammonia and nitrite levels rise, then fall back to zero as bacteria multiply.
You have two choices: wait 4 to 6 weeks before adding fish, or add a few hardy fish now and monitor the water closely. If you add fish, test the water every 2 to 3 days. If ammonia or nitrite rises above 0.5 parts per million (ppm), do a 25 percent water change when ready. Once ammonia and nitrite stay at zero for a week, the cycle is complete and the tank is ready for more fish.
Add fish gradually and watch for signs of stress
Start with a small number of hardy fish—no more than one inch of fish per gallon of water. A 20-gallon tank can hold 20 inches of fish total, but start with 5 to 10 inches and add more after the cycle is stable. Hardy fish that tolerate new tanks include tetras, danios, and corydoras catfish. Avoid delicate fish like discus or seahorses until the tank is 2 to 3 months old.
When you bring fish home, float the bag in the tank for 15 minutes so the water temperature inside the bag matches the tank. Then slowly add tank water to the bag over the next 15 minutes, a little at a time. This prevents the fish from going into shock. Finally, gently pour the fish into the tank and discard the bag water.
Watch the fish for the first week. They should be active and eating. If they hide constantly, gasp at the surface, or develop spots or torn fins, something is wrong—test the water and do a partial change if ammonia or nitrite is above zero.
Maintain the tank with regular water changes and filter cleaning
Once the tank is established, do a 25 to 30 percent water change every one to two weeks. Use the siphon to remove water from the bottom of the tank, where waste collects. This removes nitrate, which builds up over time and causes algae. Refill with tap water at the same temperature as the tank.
Clean the filter once a month by rinsing the filter media (sponge or cartridge) in a bucket of old tank water—never tap water, because chlorine kills the bacteria you need. The filter media should look dirty; that is the bacteria doing its job. You are just removing excess debris so water flows freely.
Test the water once a month after the tank is 2 months old. Ammonia and nitrite should stay at zero. Nitrate will rise slowly; if it goes above 40 ppm, do more frequent water changes. Feed fish once or twice a day, only as much as they can eat in 2 to 3 minutes. Uneaten food rots and fouls the water.
Frequently Asked Questions
Can I use tap water straight from the faucet?
Yes, for most people. Tap water contains chlorine, which evaporates or is removed by the filter within 24 hours. If your tap water smells strongly of chlorine or your local water report mentions high chlorine levels, buy a water conditioner at an aquarium store. A small bottle lasts months.
Why is my tank cloudy after I filled it?
Cloudiness is usually fine particles suspended in the water. The filter will trap them over 24 to 48 hours. If the water stays cloudy for more than 3 days, the filter may be too small or clogged. Check that water is flowing freely through the outlet.
How do I know if a fish is sick?
Sick fish hide, stop eating, gasp at the surface, or develop white spots, torn fins, or fuzzy patches. Test the water first—high ammonia or nitrite causes these signs. If water is fine, isolate the fish in a separate small tank and watch it. Many fish recover on their own if the water is clean and warm.
What should I do if a fish dies?
Remove the dead fish when ready so it does not foul the water. Test ammonia and nitrite to make sure the tank is still stable. If levels are normal, the fish may have been weak or stressed during transport. If ammonia or nitrite is high, do a water change and reduce feeding until levels drop.
Can I add decorations or plants after the tank is set up?
Yes. Rinse any new decorations with tap water before adding them. Live plants can go in anytime. Plastic plants and rocks do not affect the cycle. Rearranging existing decorations is fine and does not harm the bacteria in the filter or substrate.