Fighting goldfish

My two male goldfish seem to be more agressive to each other after I change the water in their tank. I try not to change the tank around, and have removed most of the decorations (except artificial plants), since they've been crashing into things and knocking off scales. Maybe I just notice it more after cleaning, since I'm looking at them a lot more then, and other times, they're just begging to be fed. I just turned the lights off, hoping they will calm down after a good nights sleep!

Well, I do know, that when people are trying to breed their fish, they put in a little cooler water to stilmulate rainfall and breeding season. Maybe that could be part of it?

Or maybe the cleaner water makes them feel their oats. Why not get them each a girl goldfish?

Hmmm, dkpate, I try to keep temperature and salinity the same, but i'm sure it's not perfectly matched, I didn't realize that was a natural trigger. Also, like johnarthur, I wondered if they just felt better with nice clean water. But since they're pretty big fish (8 and 12 inches) I have trouble keeping them in a big enough tank, so didn't want to add more fish to the mix. Even the 70 gal they're in now doesn't seem to give much room. Am I being too cautious? Would any girlfriends have to be similar in size?

A bunch of lady fish would sure distract the boys. Of course, you could also field an under water ball team and name them The Fighting Goldfish. What would you use for a mascot?