Setting up a fowlr or reef tank is really the same especially on a tank that size. I assume you aren't gonna be drilling the tank so you could get a hob filter. I'd get an AC 70 which gives good room for media.
You'll want to use rodi or distilled water, rodi being better as tap can lead to unsightly algae outbreaks. You'll need a test kit too, the API one works ok for the basic tests like ammonia, nitrate, nitrite and pH. If you get into corals you'll need better kits for alk, calcium and magnesium. You'll also want a refractometer to measure salinity.
To setup the tank you need to select a salt mix. Really this is up to you but I'd reccomend instant ocean as it is relatively cheap and works fine for a fowlr tank. You'll need a heater and powerhead. The powerhead should circulate the water about 15-20x per hour. So for a 20 a power head rated up to 500 gph. A lot of people like the jaebo powerheads but koralias are good too.
Then you'll need sand, and rock. You can buy live rock but dry rock is usually cheaper and becomes live over time. The sand should be aragonite based. If you want sand sitters or burrowing fish like gobies I'd reccomend a fine sand type. Stay away from crushed coral substrate.
As far as a light goes anything will work but I'd use LEDs. I'll try to find an example but really any lights work on a fowlr. You'll have to let the tank cycle but we can get to that once it is set up.
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