IMHO the best way to "cycle" a new tank FW or salt is to first start the thank with plant life (fw plants salt macro algae) then do the rest.
1) start with macro algae
2) wait a week
3) add 1 male Molly acclimated to salt. (or 1 for ever ~20 gallons or so)
4) don't add food for a week.
5) start adding very limited (1 flake per fish per day) food.
Once you get the mollies to live for a couple of weeks, the tank should support the more expensive marine only fish.
This all works because the plant life actually prefers to consume ammonia over nitrates for nitrogen. So the initial dangerous ammonia spike is much reduced or eliminated totally by the plant action. Meanwhile, the macros are also consuming co2 and returning oxygen and food (pods and plant tissue) as well. So the system actually becomes a net consumer of co2 and producer of oxygen each 24 hour period.
my .02