Fish tend to calm down when the lights are off - it seems that they just 'switch off'
However, I expect this doesn't actualy last for as long as you might think afterwards - it's just how they respond initialy. BTW, gouramies in their natural habitat are often found in swampy or 'muddy' areas with poor visibility - they don't use their eyes quite as much as might be expected considering their vibrant colors and the flaring and color changing they do when breeding/fighting. That's one reason they have those modified ventral fins - the long thread-like 'feelers' which they use to feel and taste objects. And don't forget that fish can sense movement (and therefore sound) through their lateral line.
