Both. My girlfriend and I was wondering about that too, but figured out it's the lights. This is how the tank looks with really clear water. It's incredibly clear ever since I put the skimmer to work. So far I've had to empty the crap capture 4 times in 24 hours....well, not HAD to, but there was some nasty stuff coming out. But that's just our guess....cause it does look cloudy, but when you get up to it, it's really clear...idk