Very small tanks strongly limit fish choices; focus on invertebrates or a carefully researched single centerpiece species.
Good species to investigate
- Cherry Shrimp · group 6+ · bottom zone · 18–28°C / 64.4–82.4°F
- Nerite Snail · group 1+ · bottom zone · 20–28°C / 68–82.4°F
This is a shortlist, not a recipe. Do not combine every species listed. Build one community in the interactive planner so group sizes, zone pressure and pairwise compatibility are tested together.
Why shape matters
Water volume is only one part of the picture; fish also experience length, footprint and surface area. Active schooling species need horizontal distance, bottom dwellers need floor space, and territorial fish need separable zones. The planner therefore asks for dimensions instead of using volume alone.
Leave biological margin
A new or lightly filtered 5 gallon should not be pushed to the same modeled pressure as an established, well-maintained aquarium. Stock gradually and use actual ammonia, nitrite and nitrate trends to decide whether the system is coping.