HI all. I have a small tank with a handful of tetras and my angelfish, Dave. He is about two years old but has been ill for about a week. He looks healthy with no spots or signs of damage, but he has been lolling about in the tank like he's drunk. This morning, I find him flat on the bottom of the tank. He is struggling to swim and is now facing down with his nose to the bottom of the tank. I've noticed that his stripes are constantly black, instead of fading from light to black like they normally do. The water has been changed regularly and a test didn't reveal any major issues. Can anyone suggest what may be wrong with Dave?