your goby needs
your goby needs a bigger tank. a different tank. algae waifer *pleco food* shrimp pellets sand to dig in a heater or warm water *my water with out heater is at 82 degrees even with the a/c on but i will need the heater in the winter.
your goby needs brackish water not just a little salt. if you don't have a hydrometer then mix the sea water mix at half the strength that will give you 1.010 s.g.
right now your goby need water with s.g. of 1.005 to below sea water.
if you don't want to mix the seawater buy the sea water and mix it with equal part of fresh water durring your water changes.
all the above will kill your gold fish. btw your tank is on the small side for your gold fish too.
all said with nice intentions informationally not a flame at all