if you keep bb gobies and say a fig8 puffer the salinity wont need to be over 1.010 so you wont use heaps of salt if you decide to use a marine salt mix (also being only a 50 l tank) this i feel is the safest option as you dont want to introduce ane nasties into the water. however if you decide to get saltwater from the sea i dont think i would be getting it from around the port or even tahuna as the water at the port is very polluted from industy. also be aware bb gobies really need a source of live food be this daphnia, small shrimp , other small water creatures or whiteworms (a variety is best , some will take meaty dead food but it shouldnt be relied on) but congrats on deciding to start a brackish tank with a lil planning and research it will go well