How big is he?- if over 4" you could gradually raise his tank to full marine- they live as adults in full marine. To create a brackish then get to full marine- start with a cycled freshwater tank and slowly raise the salinity- about 0.002 every two weeks (yes most people raise it that much every week but I err on the side of caution and allow the bacteria time to adjust more). You could stop at 1.010 for him if under 4". Watch for any ammonia spikes ( if spike- do a water change) and do you water changes every week (I usually do %50). You need to use marine salt- not aquarium salt. I like Instant Ocean. You need to mix the water with the salt before adding it to the aquarium. The marine salt will have instructions on the container. With the salt start with a little less than a 1/2 cup of salt per 8 gal of water.
