Mollys do not come from water that is at all similar to the water required by Bettas (Siamese fighting fish). Being very adaptable they can however survive in with Bettas if there is enough room in the tank. The basic rules for keeping fish successfully are . :- 1 inch of fish needs at least 1 gallon of water. Every tank must have a permanently running 'cycled' filter. :- Every tank must have at least 50% of its water replaced every week. A Molly is a 3 to 4 inch fish and a Betta is a 3 inch fish so the smallest sized tanks you would needto house the two species together would be a 'nominal' 10 gallon tank which will hold around 8 gallons (and a maybe a bit?) when in use.
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