FIRST of all, 20 is NOTHING to worry about. I have never had ,mine under 20(probably even 30 most the time), EVER. and the fish and most of the snails are a couple of years old.
Cigarett smoke will NOT contribute to nitrates. I am a chain smoker and the tnk is in my living room, where i spend most of my time. So, I can assure you it aint that. I even have many friends over alot who also smoke. The tank is healthy and strong, still.
Trying another kit may answer some questions. Do you use a skimmer? if so, whcih one and how big is the tank? How much LR, and What exactly have you been putting in that keeps dying? How do you acclimate them(it can take days for some things to die from improper acclimation)? Another question how about circulation? Temperature? and lighting(it can have some affect on certain inverts)?
Sorry if you have answered these Q's before, but I am just trying to help get to the bottom of your issues here.