with that many gobies there may be territorial issues and stick with only 3 fish dont try and push it yet you will get there wait out the 1 or 2 months before adding your fish trust me patients young grasshopper patients
in my 6er, I have the blue neon and gold neon. They get along. I have a red headed goby who likes to boss around every now and then. For the most part now, they all get along. Good luck 2nd time around with your tank
wow that sucks, hopefully you will find out what happened. Maybe one fish died and caused all the rest to die in the process.Just a thought.I hope this never happens to you again.