I decided against the heater. There's literally only 6 cups of water (I measured) in there. And we turned the heat on, and I'm watching his thermometer. If he gets too cold I'll put the desk lamp over him or place him on the table over the heater.
I wish fish didn't get sick so easy! Lude shed his slime coat before and it was scary bc I thought he had some freak disease. So hopefully whoever's fish is sick is just shedding their slime coat. All the others I hope they get better!
eta- I cannot believe I kept Lakitu in this .5 for months
as his "home".. It's so peeny weeny small!