A sensible stocking framework
Think in three layers instead of counting inches. Mid-water school: 10–12 small tetras, rasboras or 8 larger-bodied fish like cherry barbs. Centrepiece: one dwarf gourami, a honey gourami pair, or a single apistogramma. Bottom: 6 corydoras or 5 kuhli loaches, plus optional shrimp and snails. That three-layer pattern fills the tank visually while keeping total bioload safe—roughly 15–20 small fish depending on species.
Tested combinations
Community classic: 10 harlequin rasboras + 6 bronze corydoras + 1 honey gourami. Colour-focused: 12 ember tetras + 8 pygmy corydoras + cherry shrimp colony. Livebearer tank: 3 male platies + 3 male guppies + 6 corydoras (male-only avoids the fry explosion). Single-species showcase: 12–14 celestial pearl danios over fine sand with heavy planting. Each of these sits at a comfortable stocking level with weekly water changes.
What still does not fit
A 20 gallon is not an angelfish tank—adults reach 15 cm with tall bodies and need 30+ gallons. Common plecos, bala sharks, iridescent sharks and goldfish groups are all far too large. Tiger barbs technically fit but need groups of 8+ and nip slow tankmates, so they work best alone in this size. One fancy goldfish is defensible in a well-filtered 20, but two is pushing it.
Filtration and maintenance set the real ceiling
The counts above assume a filter turning the tank over 5–6 times per hour, weekly 25–30% water changes and modest feeding. Overfiltering a 20 gallon is cheap insurance—if your filter is rated for 30–40 gallons, the tank shrugs off missed maintenance much better. Calculate your real water volume after substrate and décor, and use that figure—not the sticker—when planning doses and stocking.