Our son has 2 ducks, and my husband added on to the chicken yard to give them space to roam when we are not outside with them. We keep ours fenced and covered because we have so many hawks in the area. Anyway we've sunk a 75 gal stock tank into the ground in there for the ducks to swim in and they love it. But it gets gross pretty quickly, Henry siphons it out at least once a week but I thought I read once that there is an additive we may be able to add to the water to help it keep it fresh. Perhaps it causes all the yuck to settle to the bottom of the tank. Does this ring a bell for anyone? I should add that when they are outside we do fill a kiddie pool for them to play in as well. But as we get busy with school we aren't home as much to let them out as often, so I want to be sure they are happy in their pen as well.