Start with the right tank size and location

The first decision is tank size. A 20-gallon tank is the practical minimum for a beginner — smaller tanks swing in temperature and chemistry too quickly, and larger ones are more forgiving. Place the tank on a sturdy, level stand away from direct sunlight, heating vents, and air conditioning units. Direct sun causes algae blooms; temperature swings stress fish.

Check that your stand can hold the weight. A 20-gallon tank filled with water, gravel, and decorations weighs roughly 200 pounds. Make sure the floor beneath it is solid — avoid placing tanks on carpet if possible, since water damage underneath becomes invisible until it's severe.

Key Takeaways

  • A 20-gallon tank is the smallest practical size for beginners because smaller tanks have unstable temperature and chemistry.
  • You need a filter rated for your tank size, a heater set to 76–78°F, and an air pump or filter that creates water movement.
  • Gravel, plants, and decorations go in before you add water, and the tank must run for 5–7 days before you add any fish.
  • Tap water contains chlorine that kills beneficial bacteria, so treat it with a dechlorinator before filling the tank.
  • Test water chemistry (ammonia, nitrite, nitrate, pH) weekly for the first month, then every two weeks once the tank is stable.

Install the filter, heater, and air system

The filter is the most important piece of equipment. Choose one rated for your tank size or slightly larger — a filter rated for 30 gallons works well on a 20-gallon tank. Filters come in three main types: hang-on-back (clips to the rim), canister (sits under the stand), and sponge (sits inside the tank). Hang-on-back filters are easiest for beginners.

A heater maintains temperature. Most tropical fish need 76–78°F. Buy a heater rated for your tank size — a 50-watt heater works for a 10-gallon tank, a 100-watt for a 20-gallon. Place it near the filter outlet so heated water circulates throughout the tank.

An air pump or air stone adds oxygen and creates water movement, which helps the filter work and prevents dead zones where waste collects. You can skip this if your filter already creates strong surface agitation — watch for ripples at the water line.

Add substrate, plants, and decorations before water

Rinse your 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. Gravel is easier to clean than sand; sand looks more natural but traps waste underneath.

Add plants and decorations now, before water. Live plants reduce algae and provide hiding spots for fish, but they need light — a basic LED light hood (12 to 16 hours per day) is enough for low-light plants like Java fern and Anubias. Plastic plants work too and require no light. Arrange rocks and wood so they won't shift when water is added.

Fill the tank with treated water and run it empty

Fill a bucket and add a dechlorinator according to the bottle's instructions. Tap water contains chlorine that kills the beneficial bacteria your tank needs. Pour the treated water slowly into the tank — place a plate on the substrate and pour onto it so the water spreads gently and doesn't kick up gravel.

Turn on the filter and heater. Let the tank run for 5 to 7 days with no fish. During this time, beneficial bacteria begin to colonize the filter media and substrate. This process is called cycling. You can speed it up slightly by adding a bottled bacteria starter (brands like Seachem Stability or Dr. Tim's One and Only follow the label), but the tank still needs several days to stabilize.

Test water chemistry before adding fish

Buy a water test kit — liquid test kits (API Master Test Kit is the standard) are more accurate than strips, though strips are faster. Test for ammonia, nitrite, nitrate, and pH. When the tank is cycling, ammonia and nitrite will spike and then drop to zero. Nitrate will rise slightly. This cycle usually takes 5 to 7 days but can take up to two weeks.

Once ammonia and nitrite read zero and the heater holds temperature steady, the tank is ready. Write down the pH and nitrate number — these are your baseline. You'll test again after adding fish to make sure chemistry stays stable.

Add fish slowly and watch for problems

Start with hardy, small fish: tetras, guppies, corydoras catfish, or danios are good choices. Add only 3 to 5 fish on the first day. Wait a week, then add 3 to 5 more if ammonia and nitrite stay at zero. Overstocking crashes water chemistry and kills fish.

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 — this prevents shock from sudden chemistry changes.

Watch for signs of stress: fish gasping at the surface, hiding constantly, or refusing food. If this happens, do a 25 percent water change (remove a quarter of the water and replace it with treated water) and test chemistry. Ammonia or nitrite above zero means the tank is not fully cycled — wait another week before adding more fish.

Maintain the tank weekly and monthly

Every week: test ammonia, nitrite, and nitrate. Do a 25 percent water change if nitrate rises above 40 ppm (parts per million). Feed fish once or twice daily, only as much as they eat in two minutes — uneaten food rots and fouls the water.

Every two weeks: clean the filter intake tube (the part that draws water in) by running it under tap water. Do not rinse the filter media itself in tap water — that kills the bacteria. Every month: clean the glass and check the heater temperature.

Every three months: test pH and make sure it hasn't drifted. If it has, a 50 percent water change usually brings it back. Replace the filter media according to the manufacturer's instructions — most last 4 to 6 weeks before they clog.

Frequently Asked Questions

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

Plan for 5 to 7 days of running the empty tank while it cycles. If you use a bacterial starter, you might be ready in 5 days. Without one, it can take up to two weeks. Test ammonia and nitrite before adding any fish — if either reads above zero, wait another few days.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and sometimes chloramine, which kill the beneficial bacteria your filter needs. Always treat it with a dechlorinator first. This takes seconds — just add the drops or powder to the water before it goes in the tank.

What if my fish die right after I add them?

The most common cause is ammonia or nitrite poisoning from an incompletely cycled tank. Test the water when ready. If ammonia or nitrite is above zero, do a 50 percent water change and wait another week before adding more fish. If chemistry is fine, the fish may have been stressed by temperature shock — always float the bag for 15 minutes before releasing them.

Do I need a light for my aquarium?

Only if you have live plants. Fish do not need light to survive, but they do need a regular day-night cycle — 12 to 16 hours of light and 8 to 12 hours of darkness. If you have no plants, you can skip the light and rely on room light, or add a basic LED hood for visibility.

How often should I change the water?

Do a 25 percent water change weekly if nitrate stays below 40 ppm. If nitrate climbs above 40 ppm, increase to a 50 percent change weekly. Never do a 100 percent water change — it removes the beneficial bacteria and crashes chemistry. The goal is to remove waste without disturbing the bacterial colony.