Sick dwarf gourami

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. Never more than 2 weeks. My water is consistently 0 ammonia & nitrites. Nitrates 5-8. My Ph ranges 8.0-8.2. Water is 80 degrees.
I have 2 F swords, 3 F platy, 2 M platy, 2 F molly (about 3 mo old), 2 F dwarf gourami, 1 M gourami and 1 F betta that I think has swim bladder that I can’t seem to cure. She lays in her log all day & only comes out to eat. I feed flakes & bug bites. So, my F & M gourami I got 3 weeks ago. The other F I got 1 week ago. With new fish if they are going to die it is around the 2 week mark. So about a week ago my M gourami started just hanging in the plants. For the past 2-3 days he just sits in the plants or on the bottom of the tank. I thought this morning he was dead but he is breathing. Just sitting on the bottom. Any suggestions or just wait & see? I have attached a picture. He looks like he is having trouble breathing

What is your water hardness. A pH of 8 and the fact that you have thriving livebearers suggests that your water may be hard. The livebearers are hard water fish. Gourami and beta are softwater fish and will always be more prone to health problems in hard water.

I do have hard water! I don’t know the levels but might have a test strip. I do know it is hard water though! Should I add a little aquarium salt?? I have nerite snails, would that hurt them?