I am using a 20 gallon tank for 3 leopard geckos (I just got a baby a few nights ago) although I plan on upgrading to a 30 gallon shortly. Temperatures seem to be well, cold side is room temp, and my hot side is about 85 degrees. I am using a red light right now but and going a purchase an under the tank soon.
A 20 gallon LONG (30 x 12 x 12) is the minimum size in which to easily establish a cool end and a warm end.
It is important to quarantine all new geckos for at least 60 days.
What sexes are your older geckos?
Geckos of different sizes should not be housed together.
Temperatures for all leos regardless of size:
88-93 F (31-34 C) ground temp at warm end inside the warm dry hide
no greater than 85 F (29.5 C) air temp - 4 inches above ground on the warm end
no greater than 75 F (24.5 C) air temp - 4 inches above ground on the cool end
Leave the UTH on 24/7. Can turn off overhead heating at night.
Without proper temperatures these leos will be unable to digest their food.
I recommend an UTH under the tank and a ceramic heat emitter overhead. Be sure to get at least one thermostat to connect to the UTH.