Brackish Tank questions

[GuiltyPleasures]

I have an established 29 gal tank that was housing much of my community fish but I've moved them to a new 150 tank except the dragon fish (violet gobie) green puffer and 3 bumble bee gobies. What I would like to do is convert that to brackish. I've been adding aquarium salt with water changes (before I had moved the community fish out) so there is still aquarium salt in there. Can I just change over to marine salt on my next water change and add that in small amounts to bring it to brackish as the fish grow or do I have to remove the fish and start the tank all over with marine salt. I also have small aquarium gravel in it. I would like to change out the gravel if needed but will leave the gravel in it if it would be okay to use. Any ideas, help or suggestions? :huh:

Thank you,
Rene

 

[Pufferpunk]

Crushed coral is my preferred substrate for BW tanks. It is easy to vacuum with a Python & keeps the pH steady, around 8. You can replace the AQ salt w/marine salt during your weekly water changes (be sure to measure with a hydrometer or refractometer). That tank isn't large enough for an adult GSP, nevermind tankmates. As the puffer matures, it will bet quite aggressive, mauling the violet goby & eating the BBGs.