Gouramis tend to be susceptible to disease, so he's probably feeling the effects of the
ammonia and/or
nitrite levels being high.
If you're planning on keeping your fish happy and healthy, I would recommend you at least invest in getting a bottle of Prime water conditioner and a liquid
test kit instead of the strips. The strips are very unreliable (which is probably why you got the weird readings for the
nitrate). I know that the liquid test kits appear to be more expensive, and they are initially, but you actually get more tests for the money out of the liquid kits than you do from the strips. A good liquid kit to invest in is the
API freshwater master test kit. Prime will detoxify ammonia and nitrite in your tank for a 24 hr period (until you can do another
water change) as well as making your water safe to add to your aquarium.
Something else to consider is the fact that you have 3 male gouramis in your tank. Sometimes gouramis can live happily together, and other times it ends badly. I'd be willing to bet that one or both of your blue gouramis is ganging up on the sick dwarf gourami, as the exact same thing happened to my roommate and I when we started a tank in our dorm room (we weren't very well informed either when we did this). Blue gouramis are actually classified as semi-aggressive fish, and your others are peaceful, which means that the blue gouramis are going to tend to bully the other fish in the tank. Plus, they're going to get about 6 inches long each, which will crowd your tank. Their attitudes and territorial nature tend to increase with age, so I'd bet that the bullying is only going to get worse with time. I'd recommend you consider finding them new homes or returning them to your
LFS for the good of your other fish.
I wish you the best of luck with your tank. I promise, it's much easier and more enjoyable after the cycle is complete and you can just enjoy your fish with minor maintenance each week.