Stunted growth

All right my night temps are hitting a low of 57 the plants are still showing growth but as you can imagine it could be more vigorous. What ways do you guys heat the growroom with out potential hazardous methods like space heaters?
 
Just an update - The bigger of the two is doing real well transplanted about a week ago into 3 gallons and she loving it check it out

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This pic was taken a few days ago she's even bigger now :hump:

And then there's this one which I'm just keeping around to see what she does hah its interesting IMG_0058.JPG
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I started another seed a week ago and that's doing well. What do you think this ones problem is tho? Too stressed at the beginning?
 
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