1. The data
The Species Ledger lists 144 freshwater species. For each one we record adult size, a cautious minimum tank size, temperature and pH ranges, temperament, minimum group size, where it swims, whether it nips fins or has long fins, the size of fish it may eat, and whether it is safe with shrimp and snails. Each species is checked against at least two public care sources, and we compile the facts ourselves. Where sources disagree we use the more cautious value and say so.
2. The rules
- Critical (−30 points, and the grade can’t be better than D): one fish can eat another; two fish share no temperature; an aggressive fish with a peaceful one; more of a species than can share a tank (for example two male bettas); bioload over 200%.
- Major (−15): a fin nipper with a long-finned fish; a tank under two-thirds of a species’ cautious minimum; bioload over 130%.
- Minor (−6): a group smaller than the species needs; a tank a bit under the cautious minimum; fish from different wild pH (captive-bred fish usually adapt); a semi-aggressive fish with much smaller peaceful ones; a narrow shared temperature; bioload over 100%.
Every tank starts at 100. Grades: A is 90+, B 80+, C 70+, D 60+, F below 60.
3. Bioload
Each fish adds “gallon units” based on its adult length: (length in cm ÷ 4)1.5, with a minimum of 0.5. A 4 cm fish counts as 1 unit and an 8 cm fish as 2.8, because waste grows faster than length. Goldfish, plecos and American cichlids count 1.5× because they are known heavy-waste fish. Capacity is your tank’s gallons, times 1.25 for an oversized filter or 0.85 for a sponge or light filter, times 1.1 with live plants. It’s a rough guide. Your real limit depends on maintenance and filtration, and water tests tell you more than any calculator.
4. Limits
Fish are individuals. Some “compatible” pairs fight and some “risky” ones live peacefully. The checker doesn’t know your water, your aquascape or your fish’s history, and it is not veterinary advice. For sick fish, ask a local fish store or an aquatic vet. If you think a rule or a fact is wrong, tell us. We fix it, date it and log it.