Aquarium simulator
Plan your stocking before the pet-store run: check compatibility, volume, water parameters.
Goldie's free Aquarium Stocking Simulator lets you plan a tank on the screen before you buy a single fish. Pick a tank size, set water parameters, add species from the catalogue, and watch the load indicator and compatibility flags update in real time.
How to use the aquarium simulator
Start by choosing a tank volume — the simulator supports sizes from a small 20-litre nano up to a 400-litre community. Then dial in temperature, pH and hardness so the water reflects what comes out of your tap or the parameters you plan to maintain. The environment comes first because everything else — what species can live there, how many, how active — depends on those numbers.
Add fish from the species catalogue one group at a time. As the stocking list grows, a coloured load bar updates and warnings appear whenever a species is kept below its schooling minimum, conflicts with another, or sits outside its water-parameter range. Move fish in and out until every indicator turns green: that's a stocking list you can confidently take to the shop.
What is fish compatibility and why it matters
Two fish that look fine next to each other in a shop tank can be a disaster at home. Compatibility is the overlap of three things: temperament, environmental needs, and behaviour. A peaceful tetra and a territorial cichlid will not share a tank for long, even at the same temperature. A fin-nipper put with long-finned bettas will leave shredded fins within a week, no matter how big the tank.
Size, schooling needs, and water chemistry matter too. A 2-cm fish is food for a 15-cm predator. Schooling species kept in groups smaller than six become stressed, hide, and stop eating. Soft-water fish in hard water slowly fail to osmoregulate. The simulator checks all of these against curated species data and flags risky pairings before you spend money — saving the most common heartbreak of new aquarists.
Understanding aquarium stocking and the 'load' indicator
Stocking is the total bioload — the amount of waste your fish produce relative to your tank's volume and filtration. Overstocking drives ammonia spikes, low oxygen, and aggression even between species that would otherwise tolerate each other. There is no universal rule: old guidelines like 'one inch per gallon' fail for tall-bodied or messy fish, and a heavily planted tank with strong filtration tolerates more than a bare setup.
Goldie's load indicator estimates bioload from species-specific biomass against your net volume, warns above 80%, and locks the build over 100%. Treat it as a planning ceiling rather than a target — staying around 60–70% leaves headroom for fish growth, occasional missed water changes, and the extra fish you will inevitably want to add. Lower numbers mean a more forgiving tank, which is what most beginners actually need.
Water parameters explained: temperature, pH, hardness
Temperature controls metabolism. Tropical species expect 24–26 °C; goldfish and white clouds want cooler water around 18–22 °C. Putting them in the same tank is a slow compromise that hurts both. pH measures acidity: tetras and corydoras from blackwater habitats prefer slightly acidic water, while African rift-lake cichlids need hard alkaline conditions to thrive.
Hardness (GH/KH) describes dissolved minerals and is what your tap water actually delivers — fighting it constantly is exhausting and risky. Pick fish whose native parameters match your tap, or learn to treat with RO water and remineralisation. The simulator compares each species' range to your set parameters so mismatches show up before you've spent any money or stressed a single fish.
Frequently asked questions
- Is this a full simulation or just a toy?
- It's an educational tool. Compatibility parameters and environment conditions come from the same data as our species cards. The simulator doesn't replace an ichthyologist's consultation, but it helps you see conflicting pairs and undersized tanks in advance.
- Where does the compatibility data come from?
- From our species cards: each entry contains compatibilityNotes verified by a reviewer. Sources — FishBase, Seriously Fish, peer-reviewed articles, and hobbyist practice.
- Can I save my aquarium?
- Yes. Once you've built it, hit 'Save' — you'll get a URL to send to yourself or a friend. There's also a PNG scene export.
- What do the colored indicators under the fish mean?
- Yellow — a warning (a schooling species kept alone, overcrowding, a mild conflict). Red — critical health drop due to water parameters.
- Is this aquarium simulator free?
- Yes, fully free. No premium tier, no login wall, no usage limit. Open the page and start planning — we fund the site with non-intrusive display ads, not by gating the tools.
- Can I use the simulator on mobile?
- Yes. The simulator is responsive and supports touch — tap a species in the catalogue to add it, drag to rearrange. On phones the controls collapse into a stacked layout; the experience is best in landscape on a tablet, but a phone works fine for quick planning.
- What does the load percentage mean?
- The load number estimates how full your tank is for the species you've added, based on biomass per fish and your tank's net volume. Below 80% you are comfortably stocked; 80–100% is the warning zone; above 100% you should expect ammonia issues and aggression. Aim for 60–70% as a healthy planning target.
Useful resources
The simulator is a handy way to test hypotheses. For expert answers, head to the guides: