Are dragon gobies, or butterfly koi fish ever been known to be aggressive at any point in time?
First I would have to ask a few questions here. How large is the tank and how long has it been up and running? You said very old gold fish are they 20 years old or older? Gold fish can live in excess of 40 ears with Koi living over 100. If the koi and the gobie were added at the same time it is very possiable the biofilter spiked causing all of the fish to become exposed to the ammonia. This will cause most fish to suck air from the surface. If there are too many fish doing this the larger ones will fight for the air. What are your water readings? Koi can and are very aggressive fish when their living space is compromised. Your Dragon is more than likely the culprit here. They are known to eat other fish Although dragons can reach lengths of over 2 feet they will rarely get 14-15 inches in a tank. Dragon gobies are actually brackish water fish that need warmer waters. And should NOT be kept in a cold fresh water tank. They can survive in a fresh warm water tank if acclamated correctly
