FW salt does not make a BW tank AT ALL! BW is in the streams between FW lakes & the ocean. There is marine salt in there & that is what you must use. You can change out the FW salt for marine, during your weekly water changes. You need a hydrometer to measure it though. The amount of salt you have in there wil probably barely register (you only have 1/2 tbsp/g). It takes roughly a cup of marine salt/5g to raise the SG to about 1.005, which is considered light BW.
