Mine used to get real stressed so I started doing them at night when they were chill, sometimes they still get stressed but not for the whole time, they just move and go back to "sleep"
Mine like to chase the bits that get sucked into the gravel siphon. They all get nose down just inches from where im siphoning off the bottom, then when something sucks into the tube they chase it as far as they can, then go right back down to where my hands are. Not in the least bit afraid. Sometimes the Oscar will bite me during a water change...no fun. Dont under estimate the power of an Oscar bite!
Yes and No, because I have induced spawning in a 2 pairs of Monos by doing a big water change. So yes they did get aggrissive because they were spawing, and No because other than that they act the same.
May just be that they feel good after a water change. Clean fresh highly oxygenated water along with a big drop in nitrates probably makes them feel pretty good.