I just bit the cheaper bullet and bought that inexpensive, folding kiddy pool while I was out running errands!
But I watched a Koi expert video and his suggestion is to simply let ones tap water trickle into the water fall. When the water level reaches maximum, use the flush cycle on the filter. He was talking about a large pond, of course so he was using several gallons an hour and based his "water changes" at 10% a
DAY! It's a different method than I've heard of and assumes that the water fall is enough agitation/spray to get the chlorine out adequately. Of course, he is in the business to sell fish and has a full-time staff to help run things! LOL! Actually found this guy very interesting and down-to-earth. , if you haven't seen them (actually, they are at that link whether you've seen them or not...). Interesting ideas on feeding, also.
BTW, I checked the box (twice!) and there were no kids included.