What's wrong with my fish/aquarium?

I'm new to keeping fish and I tried to do everything right but apparently I've screwed up. I first used a three gallon Aquaview 360 tank for my three Paradise Gourami. It had no heater or filter. They seemed to hate it so I bought a 5 gallon tetra tank with a 5-15 gallon filter and a 77 degree preset heater. I first used tap water mixed with half a teaspoon of Aquasafe conditioner and a tablespoon of Aquiarium salt. I cycled the tank for a day and then put my three fish in. They love it for a few hours but when I woke up the next morning they were all sitting at the bottom of the tank and then swimming up gasping for air. I did some research and this means too much ammonia so I bought ammonia neutralizer and drop tester as well as pH tester and changer. I tested the ammonia and it was around .25 so I used the neutralizer until it hit 0. I tested the pH and it was about 7.2 so I lowered it a bit to around 7. A day and a half later two of the Gourami were dead at the bottom of the fish tank (not belly up). I then thought that maybe the tap water was the problem since I live in an area that has terrible town water. I bought 5 gallon water jugs and repeated the process with them. The last Gourami is at the bottom of the tank now. I have tested the water and everything is normal. Temperature, pH, ammonia levels are all normal. I thought my filter might be too strong so I lowered the speed that it was pumping out water and it didn't change anything he is still at the bottom. I don't want this fish to die, can someone help me?