[KaLiB]

I'm concerned about the 5x2x2 that you are using, how thick is the glass? If it was made for freshwater, it could be on the thin side for saltwater. Does the tank come with a sump? Its usually advisable to have a sump tank(to hide filter media & install protein skimmer) and overflow for marine tanks(remove organics on the surface which would appear as a layer of oil).

If you have no plans for corals, IMO, our local tapwater is alright for fish setups. Tapwater can be de-chlorinated with anti-chlorine/chloramine from LFS for marine use(Some are made ONLY for freshwater, so its important that you check before purchase). I'm using Nutrafin Aquaplus, so far so good. You can make artificial saltwater from a combination of tapwater and artificial salt mix.

Do you know exactly what kind of coral grouper or its the polka dot grouper(老鼠班)? Groupers can produce quite heavy bioload on the system and can get very big, depending on the specie and could potentially outgrow your 5x2x2. I would advise getting a good protein skimmer to help control dissolved organics(and ultimately your nitrates).

I've kept groupers before and you could require a sandbed depending on the grouper specie. I noticed for species like orange spotted grouper and hinds, they dig holes in the sand below the rockwork to make a hiding place. You would need to have a secure rockwork, preferably using epoxy to secure the rockwork to prevent unwanted avalanches.