Freshwater Fish Tank: Setup, Cycling and First Fish

A freshwater fish tank is not ready the day you fill it. You set up the tank, let it stabilise for 24 to 48 hours (Petco), then grow the bacteria that turn fish waste into less harmful compounds. That cycling stage takes weeks, and fish should only go in once ammonia and nitrite both read zero (PetMD). Start with a few small fish and add more slowly (BRS).

There is no single best model, but there is a best direction: bigger is easier. PetMD notes that larger tanks are generally more stable and easier to maintain than small ones, because the same mistake is diluted in more water. Before you choose a size, check three things:

• Weight. Water weighs about 8 lb per gallon, so a 20-gallon tank is almost 170 lb and a 75-gallon close to 630 lb (Petco). Make sure the floor can carry it; some rental agreements limit tank size.
• Stand. Use a level, purpose-built stand. Petco warns that an uneven surface raises the risk of cracks and leaks and that a non-manufactured stand may void the tank warranty.
• Power and space. Most tanks need at least three outlets (Petco), plus a gap behind the tank for the filter.

For price context, on 25 September 2026 Waterbox listed freshwater tanks from $79.99 (6-gallon CLEAR MINI, sale price) up to $2,249 (80-gallon EDEN X), and Bulk Reef Supply listed starter kits from about $92.99. Shapes on sale include rectangle, cube, peninsula, bookshelf, tall and bow-front (BRS).

This order follows Petco's veterinarian-approved setup sheet:

• Place the tank away from windows, outside doors, heat vents and air conditioners; sunlight feeds algae and temperature swings stress fish.
• Rinse the tank, set the stand level, and fill the tank about one third to check for leaks along the bottom edge.
• Install the filter but do not plug it in yet.
• Rinse the substrate and decor and put them in. One to two cups of gravel from an established tank, or a bottled bacteria product, gives the cycle a head start.
• Fill the rest of the way by pouring onto a small saucer so the gravel is not blown around, then add water conditioner.
• Put the heater in and wait at least 20 minutes before plugging it in, so its thermostat matches the water.
• Put the thermometer at the opposite end from the heater, and let each cord dip below the socket before rising to it (a drip loop) so water cannot run into the outlet.
• Wait at least 24 to 48 hours for the temperature to hold steady and any white cloudiness to clear. Then start cycling.

Most people use tap water, treated first. Municipal water carries chlorine or chloramine, which kill fish and the beneficial bacteria a tank depends on (Tree of Life; Aquarium Co-Op). A water conditioner neutralises both; most contain sodium thiosulfate (Aquarium Co-Op).

• City tap water: add conditioner every time, at the dose on the label. Aquarium Co-Op gives 1 ml per 10 gallons as a common ratio for one type of product, but formulas differ.
• Chloramine: it does not evaporate, so letting water stand does not work; it must be treated chemically. Water with chlorine only can be left out for 1 to 5 days or aerated for 12 to 24 hours (Aquarium Co-Op).
• Well water: may not need dechlorinating, but test it for heavy metals first (Aquarium Co-Op).
• Reverse osmosis water: BRS lists dechlorinated tap water or RO water as the two ways to fill a new tank.

Plan on weeks, not days. The 24 to 48 hours Petco mentions is only the settling period before cycling begins. After that, sources give different cycling times: 4 to 8 weeks (Tree of Life), 2 to 6 weeks or more (BRS), and 4 to 6 weeks naturally or a few days with beneficial bacteria products (PetMD). The water test, not the calendar, decides.

The cycle works in three steps (Tree of Life): fish waste and food produce ammonia, one group of bacteria turns it into nitrite, and a second group turns nitrite into nitrate. Ammonia and nitrite are the dangerous ones; a spike of either in a new tank ("new tank syndrome") can burn gills and kill fish.

For a fishless cycle, BRS suggests dosing ammonia to about 5 ppm at the start, then keeping it around 1 to 2 ppm as the bacteria grow. The tank is cycled when, for several days in a row, ammonia is 0 ppm, nitrite is 0 ppm and nitrate is 5 to 40 ppm (BRS). Liquid test kits are more accurate than strips (Tree of Life).

Choose fish by adult size and temperature, not by how they look in the store. BRS advises starting with a few small fish, a handful of shrimp or one large snail, and adding more slowly so the bacteria can keep up. Most community tropical fish want 72 to 82 F (PetMD), so a heater is standard.

Before buying anything unusual, check your state's rules. Texas, for example, prohibits all freshwater stingrays, all snakeheads, several piranha genera and arapaima (Texas Parks and Wildlife). Some popular tank fish outgrow a home aquarium entirely; see our pages on the betta, the oscar and the arapaima for adult sizes.

A practical check that store labels miss: if a tag says only "assorted cichlid" or "catfish", photograph the fish and look up the likely species before you pay. FishLens names the likely species from a photo, and our species pages list the adult size to plan for.

• Keep nitrate down. Tree of Life says to keep it under 40 ppm with regular water changes; BRS aims for under 20 ppm. The lower target gives you more margin.
• Treat top-up and change water with conditioner every time; chloramine does not leave on its own (Aquarium Co-Op).
• Keep temperature steady. A thermometer at the far end from the heater shows whether heat is reaching the whole tank (Petco).
• Test when something changes: new fish, a death, cloudy water or fish gasping at the surface all call for an ammonia and nitrite test.

Research and drafting for this page were assisted by AI and checked line by line against the sources listed below. Every number names the organisation it comes from, and where sources disagree we say so instead of picking one silently. Prices carry the date we checked them (25 September 2026); they change, so confirm before you rely on them. If you find an error, tell us and we will correct it.