So i went to the pet store yesterday and was looking at the under tank heaters..The only ones they had said "for 10-20 gallon tanks" and im just curious...how can a a uth work for a 10 a 15 AND a 20? There wasnt anything on there for adjusting the heat either...

i dont get how that works...is it meant to go under the whole tank on a ten gallon? i know that sounds really stupid but how would u get the temp right without using lights?