You mean because they are different sizes? I just don't know how I can use the same sizes when one container is only like 14" long by 10" deep by like 6" tall, and the other is a 20 long tank 30"long by 12.5" deep by 13" tall. Any advice? Should I use a 6x11 heat mat on the 20 long too? Isn't that too small? Or should I use two of them on the one tank?