GuppiClear water, happy fish.
Guppi is the aquarium app that explains itself: tap the color on your test kit to log water, get reminders for water changes, cycle a new tank safely and see which fish get along before you buy.
Download on the App StoreSpecies guide
Unlimited tests free forever. No account, no ads. 7 languages.
Free aquarium calculators
Aquarium volume calculator
Work out how many liters or gallons your fish tank holds from its length, width and height, plus the real water volume and weight.
Water change calculator
How much water to change to bring nitrate down to your target, and what one or several water changes do.
CO₂ from pH and KH
Dissolved CO2 in a planted aquarium from pH and KH (carbonate hardness), with the target range of 15–30 ppm.
Aquarium heater size
How many watts your aquarium heater needs for the tank size and room temperature.
Salinity: SG ↔ ppt
Convert specific gravity (hydrometer) to salinity in ppt (refractometer) for reef and marine aquariums at 25 °C.
dKH ↔ ppm converter
Convert water hardness between German degrees (dGH/dKH), ppm CaCO3 and meq/L.
The nitrogen cycle
How a new tank becomes safe for fish.
Ideal water parameters
Healthy ranges for every tank type.
Fish compatibility
Which popular fish can live together, and why.
Popular species
Guía de especies de acuarioGuia de espécies de aquárioGuide des espèces d’aquariumAquarium-ArtenführerGuida alle specie d’acquarioAkvaryum tür rehberi
Questions
Is Guppi free?
Yes. Water tests, charts and the journal are free and unlimited, with 2 tanks, 3 reminders, the cycle coach and the species guide. Guppi Pro adds unlimited tanks and reminders, compatibility details, every calculator and backups.
Does it work with my test kit?
Yes. Tap the color that matches your tube on the built-in color cards, or type the number from strips or digital meters.
Which fish are in the guide?
123 common aquarium fish, shrimp, snails and plants, including reef favorites, written and checked by hand.
Does Guppi use AI?
No. Species data and guides are written by hand, and the compatibility checks follow clear rules you can read.