Well for a slat water tank you dont need
undergravel filter, airpump, air stone, FRESHwater test kit for salt??
You need hydrometer, heater, test kits for ammonia, nitrate, nitrite, PH
Salt, 4-5 lbs of LR
to start and some sand..and what ever filters you have..
Everyone is giving you good advice to be a responsible fish keeper... You need to follow some if not all of the advice given if you plan on having success with SW
are to totally different things..
Do some reading before you jump in... If you take short cuts you will only be spending more money in the long run and killing fish.