Corvus wrote:In future waterchanges you can use more salt per litre until an optimum density of about 1.01 is reached.
I believe a GSP of that size should be in an SG of 1.018-22 for optimal health.
You'll need a hydrometer or refractometer to measure the SG or salinity, as well as test kits that work in saltwater. The black belly is usually a sign of stress, poor fella isn't doing very well is he?

Also, because of the fungus on the tail, I would recommend treating him with Pimafix and Melafix asap.
Do not use products that instantly remove ammonia or nitrites. You'll screw up the tank for sure. If you are in the US, get some Biospira as soon as possible. It is the ONLY readily available product that has the correct bacteria needed for cycling. Having him in an uncycled tank is going to kill him, no doubt about that, not in the shape he's already in. Using water from another tank is worthless, there are only minute amounts of the bacteria needed in the water column. If you have another mature tank, use the filter media, some substrate, decor, etc., from that. The bacteria only lives on surfaces.
In the meantime, get some Prime water declorinator, which will detoxify the ammonia and nitrites so they won't kill him. You'll have to do large daily water changes until the tank cycles unless you can get ahold of some Biospira. Prime will detoxify the ammonia and nitrites but they will still be available for the cycle as needed. If you use a remover, they aren't available and your tank will never cycle.