You have all seen them the ceramic hollow bridges you adorn your fish tank with. Spaces like this are dangerous for goldfish. I've had quite a few exhaust/stress them selves to death trying to swim forward between two points they will never be able to fit into. For some reason if a goldfish can get his nose into a space (like the hollow bridge) they will keep trying to swim forward and stress/exhaust themselves to death when all the needed to do was back up 1/4 of an inch. Keep hollow ornamental objects out of the goldfish tank. Keep tank objects far enough away from the glass so that fish can swim cleanly between them. Goldfish will keep trying to swim forward into/past a place they will never be able to fit and will die trying. Not sure of he source of this self destructive behavior but when it happens your struck by how intensely stupid what the fish was trying to do was.
Glenn
Glenn