Any Help?

[Jpmc™]

I need some help.
I have 2 neon tetras in a 1 gallon, which if you read in my other topic, is only temporary for about 2 months.
I stuck my finger in their water, and it is fairly cold.

Any way to heat it up? I can't get a submersible heater, or I will have fish stew.

 

[MermaidMel]

put it on top of your main tank where the lid would be warm

 

[bagu]

turn up house/room temp?