It's either 2 males vying for dominance or they are spawning. Can't tell from that picture. If they are spawning, you should see one of them spewing the eggs into the water by the plants.I tried to put a video in, but can’t. This is just a still picture of the fish. But they swim this close together and are going in a circle very close to each other.
Well, unfortunately information was given but you are probably not going to like the answer. That Hex tank is not large enough for goldfish that size so that may be having an effect on them. The top video looks like 2 males sparring which would be about lack of space most likely. The bottom video would be what to expect if they were spawning so neither one is really conclusive. Usually with spawning tho, the tank water gets cloudy from the milt so if that's not happening, it's most likely 2 males sparring.Thank you. I’m going to try a video.