As other people have said you need to buy a hydrometer or refractometer to measure your salinity. It should be around 1.008. Your pleco and loaches will die, they do NOT appreciate salt at all. The goby will be okay for a little bit in freshwater but as he grows needs to get brackish water. As far as gobys with aggressive fish, I had one with a green spotted puffer and it got picked on all the time. Set up a larger tank and make it brackish and get some cool monos and celebes rainbows.