Start with a tank, filter, and heater—the three things every freshwater aquarium needs

A freshwater aquarium begins with three non-negotiable pieces: a tank (10 gallons or larger for beginners), a filter rated for your tank size, and a heater set to 76–78°F. You also need a substrate (gravel or sand), a light, a thermometer, and a water conditioner that removes chlorine. Everything else—plants, decorations, specific fish—comes after these basics are in place. The filter and heater run continuously, and the water conditioner goes into every water change.

Most beginners make the mistake of adding fish before the tank is ready. Freshwater aquariums need time to build beneficial bacteria that break down fish waste. This process, called the nitrogen cycle, takes 4 to 6 weeks. Skipping this step leads to dead fish and frustration. The good news: you can speed it up slightly by adding a bacterial starter product, but you still cannot skip the waiting period.

Key Takeaways

  • A working freshwater aquarium requires a tank, filter, heater, substrate, light, thermometer, and water conditioner before you add any fish.
  • The nitrogen cycle—the growth of beneficial bacteria—takes 4 to 6 weeks, and adding fish before this is complete will kill them.
  • You can test whether your tank is ready by checking ammonia and nitrite levels with an aquarium test kit; both should read zero before fish go in.
  • Water changes of 25 to 30 percent every week keep ammonia and nitrite from building up once fish are in the tank.
  • Freshwater fish fall into three broad groups—hardy community fish, mid-level fish, and aggressive fish—and mixing the wrong groups causes fighting and death.

Choose a tank size and location before you buy anything

Start with at least 10 gallons. Smaller tanks are harder to keep stable because temperature and chemical swings happen faster in less water. A 20-gallon tank is even better for a first aquarium because it gives you more room for error. Measure your space first—a 10-gallon tank is roughly 20 inches long, 10 inches wide, and 12 inches tall. A 20-gallon is about 30 inches long.

Place the tank on a sturdy, level surface away from direct sunlight and heating vents. Sunlight causes algae blooms. Heat vents and air conditioners make temperature control harder. The spot should be close to an electrical outlet (for the filter and heater) but not in a high-traffic area where the tank could get bumped. Once the tank is full, it will weigh 80 to 200 pounds depending on size, so do not plan to move it often.

Set up the tank and run the filter for 4 to 6 weeks before adding fish

Rinse the substrate (gravel or sand) under tap water until the water runs clear—this removes dust and debris. Spread 2 to 3 inches of substrate across the bottom of the tank. Fill the tank slowly with tap water, pouring onto a plate or into a cup so the water does not blast the substrate everywhere. Add the water conditioner according to the bottle's instructions—this removes chlorine that kills beneficial bacteria.

Install the filter and heater, plug them in, and let them run. The filter should produce a gentle current; if it is too strong, adjust the flow valve. Set the heater to 76–78°F and check the thermometer after 30 minutes to confirm it is working. Place the light on a timer set for 8 to 10 hours per day. Do not add fish yet. For the next 4 to 6 weeks, the filter will grow a colony of Nitrosomonas and Nitrobacter bacteria that convert fish waste (ammonia) into less toxic compounds.

You can speed this up slightly by adding a bacterial starter product (sold as "nitrifying bacteria" or "cycle starter") according to the package directions. You can also add a small piece of fish food or a pinch of pure ammonia to feed the bacteria while you wait. Test the water every few days with an aquarium test kit. When ammonia and nitrite both read zero, the tank is ready for fish.

Test your water before and after adding fish

Buy a liquid test kit that measures ammonia, nitrite, nitrate, and pH. These cost $20 to $35 and last for hundreds of tests. Do not use paper test strips—they are less accurate. Test the water every 2 to 3 days during the first month after adding fish, then weekly for the next month, then monthly after that.

Before fish go in, ammonia and nitrite should both be zero. Nitrate can be any number (it is less toxic and gets removed during water changes). After you add fish, ammonia and nitrite may spike—this is normal. If ammonia or nitrite goes above 1.0, do a 25 percent water change when ready and test again the next day. Once fish are in, the bacteria colony will grow larger to handle their waste, and ammonia and nitrite should drop back to zero within a few days to a week.

Start with hardy fish that tolerate beginner mistakes

Hardy community fish survive temperature swings, missed feedings, and minor water quality problems. Good first fish include guppies, tetras, danios, corydoras catfish, and plecos. These fish do not fight each other and do well in groups. A 10-gallon tank can hold about 10 small fish (the "one inch of fish per gallon" rule is a rough starting point). A 20-gallon can hold roughly 20 small fish.

Avoid aggressive fish (cichlids, bettas, goldfish) and delicate fish (discus, cardinal tetras, seahorses) until you have kept a tank running for at least 6 months. Aggressive fish kill tank mates. Delicate fish die if water conditions shift even slightly. Goldfish also produce far more waste than tropical fish and need much larger tanks than most people think.

When you buy fish, ask the store worker how many of each species you can safely keep together. Some fish are solitary; others need groups. Tetras, for example, are happier in groups of 6 or more. A single tetra in a tank by itself is stressed and may not eat.

Feed fish once or twice daily and change water weekly

Feed only what your fish can eat in 2 to 3 minutes. Uneaten food rots and pollutes the water. Most beginners overfeed. A small pinch of flakes or pellets per fish is enough. If you miss a feeding, the fish will be fine—they can go several days without food. If you overfeed, the water quality crashes and fish die.

Change 25 to 30 percent of the water every week. Use a siphon or aquarium gravel vacuum to remove water from the bottom of the tank, where waste collects. Refill with tap water treated with water conditioner. This removes nitrate (which builds up over time) and keeps ammonia and nitrite from spiking. Never change 100 percent of the water at once—this kills the beneficial bacteria and removes minerals the fish need.

Clean the filter and watch for common problems

The filter cartridge or sponge traps waste and becomes clogged over time. Rinse it in old tank water (not tap water, which kills bacteria) every 2 to 4 weeks, depending on how dirty it looks. If the filter flow slows down, it is time to rinse. Replace the cartridge only when rinsing no longer helps—usually every 3 to 6 months.

Watch for signs of trouble: fish gasping at the surface (low oxygen or high ammonia), fish hiding constantly (stress or poor water quality), white spots on fish (ich, a parasite), or cloudy water (bacterial bloom). Cloudy water usually clears on its own within a few days. Ich requires treatment with medication from the aquarium store. Gasping fish need an when ready water change and a test of ammonia and nitrite. Hiding fish may need more plants or decorations to feel safe, or the water temperature may be wrong.

Frequently Asked Questions

How long does it take before I can add fish?

The nitrogen cycle takes 4 to 6 weeks. You can add fish sooner if you use a bacterial starter product and test the water, but ammonia and nitrite must both read zero before fish go in. Rushing this step kills fish.

Can I use tap water or do I need special water?

Tap water is fine as long as you treat it with a water conditioner to remove chlorine. Some tap water contains chloramine instead of chlorine—check your water company's website or ask at the aquarium store. A conditioner that removes both works for either.

What if my fish die shortly after I add them?

The most common cause is ammonia or nitrite poisoning because the nitrogen cycle was not complete. Test the water when ready. If ammonia or nitrite is above zero, do a 25 percent water change and wait another week before adding more fish. The second most common cause is temperature shock—fish moved from warm store water to a cold tank die within hours. Always float the bag in your tank for 15 minutes before releasing the fish.

Do I need live plants or are plastic decorations okay?

Plastic decorations are fine. Live plants help absorb nitrate and look nicer, but they are not required. If you want plants, start with hardy ones like Java fern or Anubias that do not need special lighting or fertilizer.

How often should I do a full tank cleaning?

Never do a full cleaning. Weekly 25 to 30 percent water changes are all you need. A full cleaning kills the beneficial bacteria and crashes the tank. Clean only the glass (with a scraper or soft cloth) and rinse the filter in old tank water when it gets clogged.