Start with the tank size and location
A 20-gallon tank is the practical minimum for a beginner freshwater setup. Smaller tanks — 10 gallons or less — swing in temperature and chemistry too quickly, making fish harder to keep alive. Larger tanks are more forgiving and cost only slightly more to run.
Place the tank on a sturdy, level surface away from direct sunlight and heating vents. Sunlight causes algae blooms. Heat vents and air conditioning units create temperature swings. The spot should be near an electrical outlet but not in a high-traffic area where the tank will get bumped. A standard 20-gallon tank weighs about 225 pounds when full, so the surface must support that weight without sagging.
Avoid placing the tank against an outside wall in winter or near a window that gets afternoon sun. Both create temperature stress that fish cannot handle.
Key Takeaways
- A 20-gallon tank is the smallest size that stays stable enough for beginners, and placement away from sunlight and vents matters as much as the tank itself.
- A filter rated for at least the tank volume per hour, a heater set to 76–78 degrees, and an air stone or airstone-equipped filter keep water safe for fish.
- Substrate, plants, and decorations should go in before you add water, and the tank must run for 5 to 7 days before any fish enter.
- The nitrogen cycle — bacteria converting fish waste into less toxic forms — takes 4 to 6 weeks to establish, so start with hardy fish and do partial water changes weekly.
- Testing water for ammonia, nitrite, and nitrate tells you when the tank is ready and when problems are starting.
Choose a filter and heater that match your tank
The filter is the most important piece of equipment. It removes waste particles and grows the bacteria that keep the water safe. Buy a filter rated for at least the tank volume per hour — so for a 20-gallon tank, a filter rated for 20 gallons per hour or higher. Hang-on-back filters and canister filters both work well for beginners. Sponge filters are cheaper but noisier.
The heater keeps water at 76 to 78 degrees Fahrenheit, the range most freshwater fish prefer. A 50-watt heater works for a 20-gallon tank. Larger tanks need larger heaters — roughly 2.5 watts per gallon. Submersible heaters are easier to hide than hang-on models.
An air stone or an airstone-equipped filter adds oxygen and water movement. Many filters include this, but if yours does not, buy a small air pump and stone for about $15. Fish need oxygen as much as they need food.
Add substrate, plants, and decorations before water
Rinse the substrate — gravel or sand — under running water until the water runs clear. This removes dust that clouds the tank. Spread 2 to 3 inches of substrate across the bottom. Do not skip this step; bare glass makes fish stressed and gives beneficial bacteria nowhere to live.
Add plants and decorations next. Live plants consume nitrate (a fish waste product) and produce oxygen, so they reduce the work the filter has to do. Beginner-friendly plants include Java fern, Anubias, and Amazon sword — all grow slowly and tolerate low light. Plastic plants work too but do not reduce nitrate. Rocks, driftwood, and caves give fish places to hide, which reduces stress.
Arrange everything before adding water because moving things around after the tank is full stirs up substrate and clouds the water for hours.
Fill the tank and run it for a week before adding fish
Fill the tank slowly with dechlorinated water. Tap water contains chlorine, which kills fish and the bacteria you need. Buy a dechlorinator at any aquarium store — a bottle costs $8 to $15 and treats hundreds of gallons. Follow the bottle instructions for dosing.
Turn on the filter and heater. Let the tank run for 5 to 7 days. During this time, the filter media begins to grow the bacteria that convert ammonia (from fish waste) into nitrite, then into nitrate. This process is called the nitrogen cycle, and it takes 4 to 6 weeks to complete. Running the tank empty gives the bacteria a head start.
Check that the heater reaches the target temperature and the filter runs quietly. If the heater does not warm the water after 24 hours, it may be faulty — return it and buy another.
Test water chemistry before and after adding fish
Buy a water test kit that measures ammonia, nitrite, and nitrate. Liquid test kits (API Master Test Kit is the standard) cost $25 to $35 and last for hundreds of tests. Test strips are cheaper but less accurate.
Before adding fish, test the water. Ammonia and nitrite should read zero. Nitrate will likely be zero too, since no fish waste is in the tank yet. After you add fish, test every 3 to 4 days for the first month. Ammonia and nitrite will spike, then fall as bacteria grow. When both stay at zero for a week, the cycle is complete.
After the cycle is established, test weekly. If ammonia or nitrite ever rises above zero, do a 25 percent water change when ready and test again in 2 days. Rising ammonia or nitrite means the filter is not keeping up — usually because the tank is overstocked or the filter is clogged.
Add hardy fish slowly and feed sparingly
Start with 3 to 5 hardy fish that tolerate the ammonia and nitrite spikes of a new tank. Danios, tetras, and corydoras catfish are good choices. Avoid fancy goldfish, bettas, and cichlids for your first tank — they are pickier about water conditions.
Add only a few fish at a time, spaced a week apart. This gives bacteria time to grow and handle the extra waste. Overstocking a new tank is the most common reason fish die.
Feed once a day, in an amount the fish finish in 2 to 3 minutes. Uneaten food rots and creates ammonia. Hungry fish are healthier than overfed ones. As the tank matures and you add more fish, you can feed twice a day, but start conservatively.
Maintain the tank with weekly water changes
Once the nitrogen cycle is complete, do a 25 percent water change every week. Remove water with a siphon or bucket, then refill with dechlorinated water at the same temperature as the tank. This removes nitrate, which accumulates over time and stresses fish.
Clean the filter monthly by rinsing the filter media in old tank water — never tap water, which kills the bacteria. Wipe algae off the glass with a soft cloth or algae scraper. Do not use soap or chemicals.
Check the heater and filter every few days to make sure they are running. A failed heater can kill fish in hours during cold weather.
Frequently Asked Questions
Can I use tap water straight from the faucet?
No. Tap water contains chlorine and sometimes chloramine, both of which kill fish and the bacteria the filter needs. Always treat tap water with a dechlorinator before adding it to the tank. Dechlorinator is inexpensive and available at any aquarium store.
How long does it take before I can add fish?
You can add a few hardy fish after 5 to 7 days of running the tank empty. However, the nitrogen cycle — the process that makes the water safe long-term — takes 4 to 6 weeks. Add fish slowly during this period and test the water every few days to watch ammonia and nitrite levels.
What should I do if fish start dying?
Test the water when ready for ammonia and nitrite. If either is above zero, do a 25 percent water change right away and test again in 2 days. If both are zero, the problem is likely overfeeding, overstocking, or a failed heater. Check that the heater is on and the filter is running. Feed less and do not add more fish until the water stabilizes.
Do I need live plants?
Live plants help reduce nitrate and produce oxygen, but they are not required. A tank with only plastic decorations and a good filter works fine. Live plants make the tank easier to maintain and look more natural, but they need light and occasionally need trimming.
How often should I clean the tank?
Do a 25 percent water change weekly and rinse the filter media monthly in old tank water. Do not do a full tank cleaning or replace all the substrate — this kills the bacteria. Wipe algae off the glass as needed, but leave some on the back wall; it is harmless and fish eat it.