tang and angelfish sparring after dark

Question
I HAVE A LARGE YELLOW TANG AND A LARGE PASSER'S ANGELFISH TOGETHER IN A 150 GALLON SALTWATER AQUARIUM AND THE YELLOW TANG AND THE ANGELFISH ALWAYS SEEM TO BUMP INTO ONE ANOTHER AS THOUGH THEY ARE HAVING A DISPUTE OVER A SLEEPING SPOT,NEITHER OF THE TWO HAS OR SHOW ANY PHYSICAL MARKS BUT THE PASSERS ANGELFISH SEEMS TO CHANGE COLOR AND IN THE MORNING HAS TINY ONE SPECKS AS THOUGH IT IS ICK  BUT THEN AFTER A COUPLE OF HOURS THE SPOTS ARE GONE THIS SPARRING BETWEEN THE TWO ONLY SEEMS TO HAPPEN AT NIGHT JUST BEFORE THE LIGHTS GO OFF.IS THERE A PROBLEM THAT I SHOULD BE WORRIED ABOUT BETWEEN THE TWO OF THEM.

Answer
Hi Alaica. What you have here is a normal argument between two semi-aggressive species of fish. They may well be sparring over a sleeping area or just coincidentally having their dispute at night. As long as neither of them are coming out of the match with injuries it is safe to say that they are settling their differences relatively peacefully and you should not be too concerned over it. As for the white spot it could very well be ich. Ich can live in a tank for a very long time without ever being seen. Usually ich out breaks occur during times of stress like fighting, deterioration of water quality, acclimation etc. When one single parasite is seen it may be because the fish host's immune system is too strong to allow a full blown outbreak. During periods of darkness your pH level may fall and then rise again in the day this is completely normal however when this occurs it may stress the fish out just enough to allow a single parasite to attach to the skin. The ich parasite after it has fed then falls off the host to then complete its lifecycle in the tank. There it forms a cyst where it multiplies, hatches and goes into a free swimming stage where it must find a fish host or die with in 24 hours. Only a small percentage will succeed and in a stable, well maintained environment with healthy fish, will never become a problem.