Can sand substrate be used with a penplax undergravel filter and how thick should it be

I am looking to set up a saltwater tank with power heads in the uplift tubes.

An undergravel filter will immediately be clogged by sand. It will fall through all the little holes, so no, it won't work. Stick with the sand, but get rid of the UG filter, they just aren't very good. As long as you have one or two good power heads, you don't need to worry about filtration at all, you should be covered. Email me if you have any more questions.

Nosoop4u