dying dwarf gourami

I have him in a quarantine tank and given a little bit of aquarium salt but no sign of improvement. He's a relatively new fish; I've only had him for about a month. When I found him lying on the floor of the aquarium, I also found a dead platy hidden away. The dead platy isn't as surprising...

I am new to gourami fish. I have a community tank of livebearers and it was recommended I try some dwarf gourami. I have a 36 gal bow front with filter off the back (came with tank). I am fairly new to fish (tank was cycled the end of May 2019). My water is changed 50% every week - 1.5 weeks...