well, 6 gouramis may be too many for that tank, just as a side note. Those are Honey Gouramis I believe. Either that or dwarf Flame, but from the pics it looks more like the shape of honey's. Please post your exact parameters and what kind of testing materials do you use? When was the last time you did a water change? Was your tank cycled before you added the fish? Gouramis are pretty sensitive to water quality.
So, either you got some sick fish or something is affecting them in your tank. They also do much better with lots of plants, which will relieve stress. When you say rocks, is there anything to truly hide inside of?
It could be a combination of things, but until we know the true history of the tank it is hard to say.