Everything you need to do before your first guppies go in: tank size, cycling, heater and temperature, plants, stocking, feeding and water changes. Every number below links to its source.
By Cropita Aquatics · Information only, not veterinary advice. If your fish are sick, talk to a fish vet.
Buy the biggest tank you can reasonably fit. OATA recommends at least 45 litres (about 12 US gallons) for a small group of guppies[1], and RSPCA Australia gives 50 litres (about 13 US gallons) as the minimum for tropical fish[6]. Bigger tanks keep temperature and water quality more stable[1].
Plan for babies. Guppies are livebearers, and breeding is "almost a certainty"; a female can have 20 to 80 fry after a pregnancy of about 24 to 30 days[1]. More fish means you need more space[6].
Placement: keep the tank out of direct sunlight, away from drafts, heat sources and loud noise[1]. Water weighs about 8.3 lb per US gallon (3.8 kg per 3.8 L), so the stand and floor must be strong and level[3].
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Step 2: Add the filter, substrate, decor and plants
A filter is always essential.[1] Its media is where the helpful bacteria that break down fish waste will live[3].
Rinse gravel well before it goes in, so dust doesn't clog the filter[3]. Then add decor and plants. Guppies like plant cover plus open space to swim, and a gentle current, since guppies are small and have long fins[1]. Skip ornaments or fake plants with sharp or rough edges that can tear fins[1].
Live or artificial? RSPCA Australia suggests roughly 50% plant cover with open areas, and notes that artificial plants reduce stress about as well as real ones, while live plants also absorb waste products from the water[6]. Plants can use nitrate as food[3]. Buy plants from an aquarium store, not from the wild[6].
Lighting: fish need a day/night cycle; lights left on 24/7 stress them[3]. RSPCA suggests no more than 10 hours of light a day[6].
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Step 3: Fill with treated water and set the heater
City tap water usually contains chlorine or chloramine, which harm fish and the filter bacteria. Treat every bucket of new water with a dechlorinator (tap water conditioner) that removes both[1],[3].
Guppies are tropical fish and need a heater to stay in range all year[1]. OATA's guppy range is 20 to 28°C (68 to 82°F)[1]. RSPCA Australia suggests 24 to 26°C (75 to 79°F) for most tropical fish[6], which sits inside that range.
Plug the heater in only after the tank is full; most heaters are glass tubes that may shatter if they aren't surrounded by water[3]. Keep a separate thermometer in the tank so you can check the heater is doing its job[3].
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Step 4: Cycle the tank before adding fish
What "cycling" means. Fish give off ammonia. One group of bacteria turns ammonia into nitrite, and a second group turns nitrite into nitrate, which is much less toxic[5]. In a tank, those bacteria live in the biofilter, and they need oxygen to work[5].
How long it takes. UF/IFAS says a new biofilter needs six to eight weeks to build up enough bacteria[5]; the Merck Veterinary Manual says it can take up to 8 weeks in a tropical tank[2], and its pet-owner guide says the bacteria develop on their own in 4 to 6 weeks[3]. Plan on several weeks.
Fishless cycling, step by step. The Merck Veterinary Manual describes it this way: set the tank up completely but add no fish, then add ammonia to reach at least 1 to 5 mg/L. Test regularly for ammonia, nitrite and nitrate. Once no ammonia or nitrite is present, fish can be added safely.[2]
Test often while the tank is new. Merck recommends daily testing during the startup of a new biofilter[7]; RSPCA suggests testing ammonia and nitrite every two to three days for the first six weeks[6].
Why it matters. Skipping the cycle can lead to "new tank syndrome", usually within the first 6 weeks: ammonia or nitrite builds up and fish become lethargic, stop eating and often die[2]. OATA explains that it happens when there aren't enough bacteria to handle the fish's waste[1].
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Step 5: Check your water numbers
Test kits are a necessity, not an extra[1]. OATA's targets for guppies[1]:
Ammonia
0 mg/L
Nitrite
0 mg/L
Nitrate
No more than 20 mg/L above your tap water
pH
7.0 to 8.0
General hardness
8 to 18 °dH (medium-hard)
Temperature
20 to 28°C (68 to 82°F)
Once the tank is running, OATA recommends testing at least once a week, especially after adding fish[1].
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Step 6: Add guppies slowly
OATA suggests keeping at least three guppies together, in a ratio of two females to each male, to reduce stress on the females[1]. Add fish slowly: the bacteria need time to catch up every time more fish (and more food) go in[1]. Merck's pet-owner guide suggests 1 or 2 fish at a time[3].
Quarantine. The Merck Veterinary Manual calls 30 days the minimum quarantine period, and describes a simple home quarantine setup: a 10-gallon tank with a sponge filter, small air pump and heater[2].
Choosing healthy fish. Look for clear, bright eyes, undamaged fins, intact scales, normal swimming and steady breathing, and don't buy from a tank that has sick fish in it[1].
Acclimating. OATA's floating-bag method: lights off, float the bag for 10 minutes so the temperatures match, then add small amounts of tank water to the bag over up to 20 minutes, and release the fish with as little bag water as possible[1].
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Step 7: Feed small amounts once a day
Guppies are omnivores. OATA recommends a good-quality flake or granule with a high algae content, plus small live or frozen foods such as daphnia or small bloodworm as extras[1].
How much: only what they can eat within a few minutes, once a day[1]. RSPCA puts it as "under two minutes"[6]. Remove uneaten food[4], because it breaks down and releases waste into the water[1]. RSPCA calls overfeeding the most common problem with all fish[6].
Merck suggests buying dried food in small amounts and keeping it on hand for no more than 2 months[3].
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Step 8: Change water and maintain the tank every week
OATA strongly recommends a 25% partial water change at least once a week[1]; RSPCA suggests 10 to 25% weekly, combined with a gravel clean[6]. Merck's maintenance schedule lists a water change every 2 weeks, with the amount depending on how many fish you keep[4]. This guide follows OATA's weekly schedule.
Use a gravel siphon to pull out waste. Never start a siphon by mouth[1],[6].
Treat the replacement water with dechlorinator before it goes in[1],[4].
If the filter clogs, rinse the media lightly in the tank water you just removed, never under the tap, so you don't kill the helpful bacteria[1],[6].
Every day: check the fish, the water temperature and the equipment[4].
OATA recommends at least 45 litres (about 12 US gallons) for a small group of guppies, and RSPCA Australia gives 50 litres (about 13 US gallons) as the minimum for tropical fish. Bigger tanks stay more stable, and guppies breed readily, so more room helps.[1],[6]
How long does it take to cycle a new guppy tank?
Usually several weeks. UF/IFAS says a new biofilter needs six to eight weeks, and the Merck Veterinary Manual says up to 8 weeks for a tropical tank. The tank is ready when ammonia and nitrite both test at zero.[5],[2]
What temperature do guppies need?
OATA gives 20 to 28°C (68 to 82°F) for guppies, and a heater is needed to hold that range all year. Use a separate thermometer to check it.[1],[3]
How often should I change the water in a guppy tank?
OATA recommends a 25% partial water change at least once a week, with the new water treated with dechlorinator. RSPCA Australia suggests 10 to 25% weekly.[1],[6]
How much should I feed guppies?
Once a day, only as much as they can finish within a few minutes. Remove leftovers, because uneaten food breaks down and pollutes the water.[1],[4]
How many guppies should I start with?
OATA suggests at least three, in a ratio of two females to each male, added slowly so the filter bacteria can keep up.[1]
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