Will a betta fish kill a guppy fish?

Bettas have a lot more personality than guppies, and require less space (5 gallon+). However, I have had boredom and tail biting issues with mine, so I had to upgrade to a 10 gallon tank with pygmy cories to stop him being bored. This is a risky move as many bettas are aggressive and won't accept tank mates.

Guppies require 10 gallon+ tanks, and are stunning creatures. However, they are less intelligent and have less personality than bettas.

Both fish require a heated and filtered tank, and no matter what you decide it is of upmost importance that you do a fishless cycle to create a healthy bacterial colony before you add any fish. Without doing this your fish will be in pain from ammonia and nitrite spikes and may die.