is this normal for fish to do after a partial water change?

[Candi]

I have a 55 galon tank with mollies, sordtails and guppies. I used a gravel vaccum thing and sucked out 1/2 the old water and added fresh water to fill the tank back up as I usually do. I done this 2 days ago. Today, I have babies everywhere and it seems most of the adult fish are swimming at the top of the tank and not very actice. I don't think I have really paid attention before so I was just wondering if this is normal? They are still eating and look fine, just not as active and staying near the top of the tank. I have maintained the same temp and have plenty of air in the tank with a bubble curtain all the way across the back, underground filters and 2 large filters on the back of the tank too. I don't usually seperate the fry and adults, and it seems they don't pay much attention to the little ones, maybe because there is so many sizes of them in there, but they have plenty of hiding places and greenery so I usually sell some to the local fish store.