How to heat betta jars?

How can many betta jars be heated cost efficiently?

 

Jars? Bettas should be in a 2.5g tank minimum.

Other than that, you could divide a large tank instead of having to heat individual ones, or keep them all in a warm fish room.

 

How big are the jars? And how many? Are they for fry?

 

Assuming the OP is breeding, I would use heat tape. If it's a small amount you can just put them in a tub full of heated water.

 

It's for fry
If I put jars in a plastic tub with a heater would the heater melt the plastic?

 

I've done it in a pinch before and it didn't melt it, it was even resting on the bottom of the tub.

 

I've used heaters in plastic tubes but I put rocks/gravel/sand something on the bottom so the heater didn't touch the plastic if it fell or got knocked down(used it for housing/qt adult/fry cichlids).