Yet another noob question. I'll have tons until I get it right. I might even ask the same questions a million times until it clicks in this brain of mine, so please forgive me. Thanks!

What exactly IS ambient temperature?
What exactly IS ambient lighting?

I have an UTH. Should I get the hydrofarm and attach it to the UTH or should I get a CHE? Both are about the same price.

Is it okay to leave an infrared light on all the time? I have a 15 gallon tank. I am trying to get all our temps up to par and stabilized.

I don't have the sensors in the right spot, will be fixing those today, but temp. at top of tank is 70o on cool side and 90o-100o on hot side. So I know temps at bottom of tank are way off. How can I fix this?