Compare their water ranges
Betta fish: 24–28°C, pH 6–7.5. Fancy goldfish: 18–24°C, pH 6.5–8.5. A shared number is only a starting point; stable conditions and provenance still matter.
Plan for adult size and space
Use at least the larger baseline of 150 litres and 100 cm swimming length, then add margin for both animals’ full social groups, territory and waste.
Behaviour can override the numbers
Betta fish is described here as territorial; Fancy goldfish as social. Introductions need observation and a ready separation plan. This assessment is not a guarantee of individual behaviour.