I agree--get a small heater, and move the light so it doesn't affect the temp of the tank. I have a betta in my planted 40, and he swims all over in the tank. They are tropical, so 60 is not an appropriate temp for the fish, and the fluctuations from the light going on and off won't help, and may stress the fish.
As for oxygen levels--bettas are labyrinth fish. They get very little of their oxygen from their water. Instead, they surface and gulp air into the labyrinth organ within their heads, and get oxygen from the air. This is why bettas is the plant jars sometimes die--the plant roots over grow the surface and prevent the betta from being able to surface.
As for oxygen levels--bettas are labyrinth fish. They get very little of their oxygen from their water. Instead, they surface and gulp air into the labyrinth organ within their heads, and get oxygen from the air. This is why bettas is the plant jars sometimes die--the plant roots over grow the surface and prevent the betta from being able to surface.