Those are called Ventral Fins, I really don't know what their suppose to be used for but when I had mine I noticed they would use them like feelers. They would reach out and touch each other and other things with them.
I know that antennas in Catfish are for food detection, but I am wondering myself what they're for in Gouramis. If they're not for food detection then they must be some kind of "feelers", as Gunnie said. Maybe they help them feel the environment they're in.
Yes a GouramI uses those feelers to help sense out his environment and feel distance (like a cat uses his whiskers) they often come from Blackwater rivers which are stained brown from dead leaves and peat - the feelers help him see- they also use them to spare with each other (like a sword fight but no damage is done).
I had a gouramI that lost one of its feelers (it only had one at the LFS but I bought it anyway) and it took a very long time to grow back but it was never the same as the other one. Other nippy fish find their feelers very appetizing and will try to eat them.
The are feelers. My dad has breed many varieties for many years and he tells me that they exude little electrical charges - they send out a little signal which bounces back to the fish to let them know distances, tensions from other fish, etc. They are amazing to watch when they are using them. I have only the dwarf variety now but do love them. Yes they can be aggressive, but they can also be very sweet and innocent. When I drop my water I use a product which is called Aqua Septic - it a preventative. I'm not saying it's good for you now, but perhaps when your problems are ironed out..... also if you can, quarantine your little buddy so that he has the best chance of getting better while he is vulnerable.
Their feelers are pretty cool, how they sense their environment. They get into a kind of contest with my ghost shrimp. The ghost shrimp started feeling the GouramI up every time it swam by, and now the GouramI returns the favor. They use them is murky water, which is why they prefer to have abundant floating cover, or floating vegetation so they can hide themselves.