I have aquarium fish, molly, dalmatian, guppies, molly fry,snail, algae eaters.

whats the best way to change the water?

I would scoop out cup fulls of water calmly and carefully. Then just mix fresh new water in afterwords. Try not to terrify the fish.

U can't change it 100 percent. Maybe 50 then slowly add water back in. If u change it too quick you put the fish in shock because of temp ph etc... and they will die.