Start with the right tank size and shape for your space
The tank itself is your foundation, and bigger is easier than smaller. A 20-gallon tank is the practical minimum for a beginner because it holds temperature and water chemistry more steadily than a 10-gallon. Smaller tanks swing wildly when you change the water or add food, which stresses fish and kills them faster. A standard 20-gallon is 24 inches long, 12 inches wide, and 16 inches tall — it fits on most furniture without special support.
Glass tanks cost less than acrylic but scratch more easily and are heavier. Acrylic is lighter and won't shatter, but it scratches from cleaning and yellows over time. For a first tank, glass is the better choice. Avoid anything smaller than 10 gallons unless you are keeping only a betta fish, which can survive in smaller water but will not thrive.
Check that your stand is rated for the weight. A filled 20-gallon tank weighs roughly 170 pounds. An undersized stand will bow or collapse. Most aquarium stands sold together with tanks are built for that weight, but if you use a dresser or shelf, verify the weight limit before you fill it.
Key Takeaways
- A 20-gallon glass tank on a proper stand is the most reliable starting point because it resists temperature swings and fits standard furniture.
- You need a filter rated for your tank size, a heater to hold temperature steady, and an air pump or powerhead to move water — all three work together to keep fish alive.
- Gravel or sand substrate goes on the bottom, then rocks and plants create hiding spots that reduce stress and keep fish from dying of fright.
- Tap water contains chlorine that kills fish; you must treat it with a dechlorinator before adding it to the tank, and you should test the water before adding fish.
- Let the tank run for one to two weeks before adding fish so bacteria can colonize the filter and break down waste.
Install filtration, heating, and aeration before adding water
A filter removes waste and dead matter from the water. The filter size must match your tank — a 20-gallon tank needs a filter rated for at least 20 gallons per hour (GPH). Hang-on-back filters are cheap and straightforward to clean; canister filters cost more but run quieter and take up no tank space. For a first tank, a hang-on-back is sufficient.
A heater holds the water at a steady temperature. Most tropical fish need 76 to 78 degrees Fahrenheit. A 50-watt heater works for a 10-gallon tank; a 100-watt heater works for a 20-gallon. Buy a heater with a built-in thermostat so it turns on and off automatically. Without a heater, your tank temperature will swing with the room, and fish will sicken or die.
An air pump or powerhead moves water and adds oxygen. Many filters do this already, but if your filter is quiet and still, add an air pump with an airstone. The bubbles break the water surface and let oxygen in. Stagnant water kills fish even if the filter is running.
Install all three before you add water. Place the filter intake tube and heater in the tank, plug the power cords into a power strip (not directly into the wall), and test that everything runs. Then turn it off and prepare the substrate.
Add substrate, rocks, and plants to create a livable environment
Substrate is the material on the bottom of the tank. Gravel is standard and costs little; sand looks more natural but traps waste. Rinse gravel thoroughly under running water until the water runs clear — unwashed gravel clouds the tank for days. Spread 2 to 3 inches across the bottom.
Rocks and plants give fish places to hide and reduce stress. Stressed fish stop eating and die. Use rocks that are smooth and won't cut fish; avoid anything with sharp edges. Live plants are better than plastic because they absorb nitrate (a fish waste product) and produce oxygen, but they need light and nutrients. For a first tank, hardy plants like Java fern or Anubias are nearly impossible to kill. Tie them to rocks with fishing line rather than burying them, or they will rot.
Arrange rocks and plants before you fill the tank so you do not have to reach in and disturb the substrate later. Leave open space for the fish to swim and for you to see them.
Treat tap water and fill the tank slowly
Tap water contains chlorine and chloramine, which kill fish and the bacteria you need in the filter. Buy a dechlorinator — a liquid or powder you add to the water before it enters the tank. Follow the bottle instructions for dosage. Some dechlorinator works when ready; others take a few minutes.
Fill the tank slowly by pouring water over a plate or rock so the current does not scatter your substrate. This takes time but keeps the gravel in place and the water clear. If you rush and pour directly, you will spend hours waiting for the water to clear.
Once the tank is full, turn on the filter and heater. Check that the heater is submerged and the filter intake is drawing water. Let the tank run for one to two weeks before adding any fish. During this time, beneficial bacteria colonize the filter media and begin breaking down waste. If you add fish before this happens, their waste will poison them.
Test water chemistry before adding your first fish
Water chemistry matters. Fish produce ammonia as waste; bacteria in the filter convert ammonia to nitrite, then to nitrate. If ammonia or nitrite is high, fish die. Nitrate is less toxic but still needs to be kept low by water changes.
Buy a test kit — a liquid kit is more accurate than strips, though strips are faster. Test for ammonia, nitrite, nitrate, and pH. After one to two weeks of running, ammonia and nitrite should be zero or very close. Nitrate will be low but not zero. If ammonia or nitrite is still high, wait another week and test again.
pH is how acidic or basic the water is. Most fish tolerate a pH between 6.5 and 7.5. If your tap water is outside this range, you may need to adjust it, but for a first tank, accept what comes from the tap and choose fish that match it.
Stock the tank gradually with fish suited to your setup
Add fish slowly — no more than three or four small fish at a time, and wait a week between additions. This lets the bacteria in the filter grow to handle the extra waste. If you dump ten fish in at once, ammonia spikes and they die.
Choose fish that are hardy and small. Tetras, danios, and corydoras catfish are forgiving. Avoid large fish like goldfish or plecos unless your tank is much bigger — they produce enormous amounts of waste and need frequent water changes. Avoid aggressive fish like cichlids if you are new to this; they fight and kill tank mates.
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 open the bag and let the fish swim out slowly. Do not dump them in; the shock can kill them.
Change water weekly and clean the filter monthly
A 25 percent water change once a week keeps nitrate from building up. Siphon out a quarter of the tank using a gravel vacuum, which pulls waste from the substrate as it drains. Treat the new water with dechlorinator before adding it back.
Clean the filter media monthly by rinsing it in old tank water that you removed during the water change. Never rinse it under tap water — chlorine kills the bacteria you need. Squeeze the media gently until the water runs clear, then put it back in the filter.
Test the water every two weeks. Once your tank is stable, ammonia and nitrite should stay at zero. If either rises, do a larger water change and check that the filter is not clogged.
Frequently Asked Questions
Can I use tap water straight from the faucet?
No. Tap water contains chlorine and chloramine that kill fish and beneficial bacteria. You must treat it with a dechlorinator before adding it to the tank. Follow the dosage on the bottle — most work within minutes.
How long does it take before I can add fish?
One to two weeks. The filter needs time for beneficial bacteria to colonize the media so they can break down fish waste. If you add fish before this happens, ammonia and nitrite will spike and poison them. Test the water before adding fish to confirm ammonia and nitrite are at zero.
What size tank do I really need?
A 20-gallon tank is the practical minimum for most fish because it holds temperature and chemistry steadier than smaller tanks. Smaller tanks swing wildly and stress fish. If space is tight, a 10-gallon works, but you will have fewer fish and more frequent water changes.
Do I need live plants or can I use plastic?
Plastic works, but live plants are better. They absorb nitrate and produce oxygen, which keeps the water healthier. Hardy plants like Java fern and Anubias are nearly impossible to kill and need only moderate light. If you want no plants at all, you can make it work with more frequent water changes.
What happens if ammonia or nitrite stays high after two weeks?
The filter may not be running properly, or the tank may be overstocked. Check that the filter is drawing water and the intake is not clogged. Do a 50 percent water change to lower ammonia and nitrite when ready, then wait another week and test again. If it is still high, remove some fish or add a second filter.