Start with a tank, filter, and heater — the three things that keep fish alive

A working aquarium needs three things: a container that holds water without leaking, a filter that removes waste, and a heater that keeps the temperature steady. Everything else — plants, decorations, lighting — makes the tank look better or gives fish places to hide, but those three do the actual work. You can build a basic aquarium in a few hours with equipment from any pet store or online retailer.

The tank itself can be glass or acrylic. Glass is cheaper and doesn't scratch, but it's heavy and breaks if you drop it. Acrylic is lighter and tougher, but costs more and scratches easily if you clean it with the wrong cloth. For a first tank, a 20-gallon glass aquarium is a good size — large enough that the water stays stable when you miss a water change, small enough to fit on a stand in most rooms.

Key Takeaways

  • A filter, heater, and tank are the only equipment you must have; everything else improves the tank but isn't required for fish to survive.
  • Gravel or sand on the bottom gives bacteria a place to live and helps the filter work, so don't skip the substrate even though it seems optional.
  • Tap water contains chlorine that kills fish, so you must treat it with a dechlorinator before adding fish, or let it sit for 24 hours first.
  • The tank needs to run for one to two weeks before you add fish, so the filter bacteria can establish and the water can stabilize.
  • A basic setup costs between $100 and $300 depending on tank size and whether you buy new or used equipment.

Choose a tank size based on where it will sit and what fish you want to keep

Tank size matters because larger tanks are more forgiving — the water stays cleaner longer, temperature swings are smaller, and you have room for more fish. A 10-gallon tank is the smallest size that works for most fish, though it requires more frequent water changes. A 20-gallon tank is easier to maintain. A 40-gallon or larger tank is even more stable but takes up more space and costs more to fill and heat.

Before you buy, measure the space where the tank will sit. A 20-gallon tank is 30 inches long, 12 inches wide, and 12 inches tall. A 40-gallon breeder tank is 36 inches long, 18 inches wide, and 18 inches tall. The stand underneath must be sturdy — a full 20-gallon tank weighs about 225 pounds. Check that the floor can handle the weight and that the stand won't tip if someone leans on it.

Different fish need different tank sizes. Bettas can live in a 5-gallon tank if it has a filter and heater, though they do better in 10 gallons or more. Goldfish need at least 20 gallons for the first fish and 10 more for each additional fish. Tetras and other small community fish can live in a 20-gallon tank with 10 to 15 fish. Look up the specific fish you want before you buy the tank.

Add substrate, then install the filter and heater before filling with water

Substrate is the material on the bottom — usually gravel or sand. It gives the filter bacteria a place to live and makes the tank look finished. Rinse the substrate in a bucket under running water until the water runs clear, which takes 10 to 15 minutes. This removes dust that would cloud the water. Spread the rinsed substrate evenly across the bottom of the tank, about 2 inches deep.

The filter sits inside or outside the tank depending on the type. An internal filter (also called a sponge filter or hang-on filter) clips to the side or sits on the bottom. An external filter (canister filter) sits under the stand and connects to the tank with hoses. For a first tank, a hang-on filter is simpler — it costs less and takes up less space. Follow the manufacturer's instructions to install it, but do not turn it on yet.

The heater is a stick or tube that plugs into an outlet and sits in the tank. Most heaters have a dial to set the target temperature. For tropical fish, set it to 76 to 78 degrees Fahrenheit. For goldfish or other cold-water fish, you may not need a heater at all if your room stays above 65 degrees. Install the heater in a spot where water can flow around it — not in a corner where it might overheat a small area.

Fill the tank slowly and treat the water before turning on the filter

Fill the tank with tap water from a hose or bucket. If you use a hose, run it for a few seconds first to clear any debris. Fill slowly so the substrate doesn't cloud up. Stop when the water is about an inch below the top of the tank. The water will look cloudy at first — this is normal and will clear in a few hours or a day.

Tap water contains chlorine and sometimes chloramine, which kill fish and the bacteria the filter needs. You have two choices: add a dechlorinator product (follow the bottle's instructions for how much), or let the water sit for 24 hours so the chlorine evaporates on its own. Dechlorinator works when ready, so you can turn on the filter right away. If you wait 24 hours, the tank will be ready sooner but you'll have to wait before adding fish.

Once the water is treated, turn on the filter and heater. The filter will make noise and bubbles at first — this is normal. Let it run for one to two weeks before you add any fish. During this time, bacteria will colonize the filter media and the water will stabilize. You can add plants and decorations during this waiting period.

Test the water and do a partial water change before adding fish

After one to two weeks, the tank is ready for fish. Before you add them, test the water with a test kit. You're looking for ammonia (should be 0), nitrite (should be 0), and nitrate (should be below 20). These kits come as strips you dip in the water or as liquid tests where you add drops and watch the color change. Liquid tests are more accurate but take longer.

If ammonia or nitrite are above 0, wait another week and test again. If they're still high after three weeks, do a 25 percent water change (remove a quarter of the water and replace it with treated tap water) and test again in a few days. High ammonia or nitrite means the filter bacteria haven't fully established yet.

Once ammonia and nitrite are both 0, do a 25 percent water change to remove any waste that's built up, then add your first fish. Start with just a few — adding too many at once will spike ammonia and nitrite again. Add more fish gradually over the next few weeks as the filter adjusts.

Maintain the tank with weekly water changes and monthly filter cleaning

Every week, remove about 25 percent of the water and replace it with treated tap water. This removes waste that the filter can't catch and keeps nitrate levels down. Use a siphon or a bucket — a siphon is faster but a bucket works fine. While you're doing the water change, wipe algae off the glass with a soft cloth or algae scraper.

Every month, rinse the filter media in a bucket of old tank water (not tap water, which will kill the bacteria). Squeeze it gently until the water runs clear. Do not use soap or hot water. This removes trapped waste without destroying the bacteria that make the filter work. If the filter has a mechanical pre-filter (a sponge or cartridge that catches large particles), replace it every month or two.

Check the heater temperature once a week to make sure it's still set correctly. Clean the glass and decorations as needed. Test the water once a month to catch problems early. If ammonia or nitrite spike, do a 50 percent water change when ready and check that the filter is running and the heater is working.

Common mistakes that kill fish or waste money

The biggest mistake is adding fish before the filter bacteria establish. This causes ammonia and nitrite to spike, which stresses or kills the fish. Wait the full one to two weeks even though it's tempting to add fish sooner. The second mistake is overstocking — adding too many fish at once or buying a tank that's too small for the fish you want. Research the fish first, then buy the tank.

The third mistake is not treating tap water. Chlorine kills fish when ready, so always use a dechlorinator or let the water sit 24 hours. The fourth mistake is cleaning the filter with tap water, which kills the bacteria and forces you to wait weeks for the tank to cycle again. Always rinse the filter in old tank water.

The fifth mistake is not doing water changes. Waste builds up and nitrate climbs, which stresses fish and causes algae blooms. Do a 25 percent water change every week without fail. The sixth mistake is buying a tank without a stand, then putting it on a bookshelf or dresser that can't handle the weight. A full tank is heavy — use a proper stand.

Frequently Asked Questions

How much does it cost to set up a basic aquarium?

A 20-gallon tank with filter, heater, substrate, and stand costs between $100 and $200 new. Add $20 to $40 for a test kit and dechlorinator. Used equipment is cheaper — you can find complete setups on Facebook Marketplace or Craigslist for $50 to $100. Check that the tank doesn't leak and the filter still works before you buy used.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine that kills fish and filter bacteria. Use a dechlorinator product (follow the bottle for dosage) or let the water sit uncovered for 24 hours so the chlorine evaporates. Dechlorinator is faster and costs a few dollars per bottle.

How long do I have to wait before adding fish?

One to two weeks. The filter needs time for bacteria to colonize the media. If you add fish before this happens, ammonia will spike and kill them. You can tell the tank is ready when an ammonia and nitrite test shows 0 for both.

What if my tank gets cloudy after I fill it?

Cloudiness is usually bacteria bloom or suspended particles from the substrate. It will clear on its own in a few hours to a day. Do not do a water change — this will restart the cycle. If it's still cloudy after three days, do a 25 percent water change and check that the filter is running.

Do I need plants or decorations?

No. Plants and decorations are optional. They make the tank look better and give fish places to hide, which reduces stress. But fish can live in a bare tank with just substrate, a filter, and a heater. Add plants and decorations after the tank is established if you want them.