 When they're done feeding, if they hadn't eaten all their food and the water starts to look cloudy in the slightest bit, then I would do a complete or at least a good percentage partial water change. You could dip water out of a local pond or lake. Or if you just want to use tap water, you could have a couple different containers or buckets. Even if it comes from a well, you might want to do this. Just pour it ahead of time and let it sit for a couple days, particularly if it's coming out of a tap.