yes I mean one pool per 50 birds.
do not think there is a filter big enough to keep any size pond clean,ducks are verry messy,at first you try to keep clean but you will give up.
Also use the little giant waters 1 per 100 birds,clean them twice a day,trick with waters is make a rock base around it mine are about 4'.ducks will drink and preen(sp) at any water they find then they will dig holes,if you have enough rocks wide enough around the water it will stop them.I'll try finding some pic's.
I've tried the bigger stock tanks and for me it just too much to water to waste,500 gallon tanked cleaned evey day is alot of water.Kiddy pool are cheap 9bucks last a year or so are easy to move to a new area after cleaning.Would not worry if you can't clean as much as I do there are times when mine don't get cleaned evey day and they do not seem to care,most times when I do clean the older ducks will go for the puddles on ground(from cleaning) then the fresh water in pool.
Now geese are diffrent or at least the one I had,he was in a dog kennel by himself as I was training him(clicker)for a friend he had a rubber 6 gallon water and a water bowl 1' high to keep him out of for fresh water he never drank the clean water and ever day would get clean water in his tub,he would not drink untill he pooped 2-3 times in it.
I'm a big in do the best you can,but if you have friends over and you feel bad about the way things look or smell you have to do better for your animals.
good luck