While plants produce oxygene while processing photosythesis during periods of "lights on", plant consume oxygene over periods with "lights off".
If the tank contains enough plants in a comparable "small amount" of water, the oxygen consumtion over night could reduce the oxygene of the water to zero. This would cause the fishes to die. (Maybe bigger ones faster than smaller ones...)
High temperatures can increase this effect, because cold water has a better capability to store oxygene than warm water. But in general LED light does not warm up waters like for example common light bulbs would do.