there is absolutely no reason to let the water sit if you use a water dechlorinator. the only reason you MIGHT let it sit with now fish is if you didn't have a dechlorinator and had chloramines in the water, which take about a week to break down and gas off.
some might say you have to let it sit and "cycle" before adding fish, but this is false, because a cycle only takes place once the fish are added. a cycle is the process by which good, nitrifying bacteria grow a colony in your pond/aquarium once food has been introduced. these bacteria are present in the water at all times, but in very, very tiny ammounts. when ammonia from fish waste is introduced to them, they begin to establish a colony on the hard surfaces in the tank, usually the filter, but also in the gravel and on the walls,
plants, and rocks. the first bacteria eat the ammonia and process it into nitrIte. both ammonia and nitrite can and will kill fish if allowed to get too high. that's why lots of water changes are necessary. once the nitrites start rising, a second type of bacteria will begin to establish and convert the nitrite into nitrAte, which is much less hamful.
plants will use nitrAte for food (it is the first number in all fertilizers, nitrogen) and if the nitrate gets to 0ppm you may have to start adding fish-safe aquatic
plant fertilizers.
in a fully cycled pond/aquarium there should be 0 ammonia, 0 nitrite, and less than 30ppm nitrate. now, even though the
plants use nitrate, you STILL have to do regular water changes. in a large enough pond, you may only have to do them once a month, or once every two months. the smaller and more heavily stocked the container, the more water changes you have to do.
also, NEVER throw out the filter sponges. that is where your bacteria live. if you replace them, you have to start all over.
i don't usually let my water sit... but i do have very good well water. when i lived in an apartment, i used to let it sit 24-48 hours.
aren't all kids? mine sure are! glad the fish are making it 