I think you shouldn't do a water change whilst doing the treatment (will dilute the treatment) for planaria, which is a week I think, then you can do one, Tuesday/Wednesday. By then there shouldn't be any planaria left so I don't think I would do another treatment at this stage, but at least you have it to hand should it recur at some point in the future.
As grubs points out about the eggs, and you had very few hydra anyway, I would avoid doing a large water change though, as that can cause a shrimp to moult and you don't want that happening to the one carrying the eggs!
Simon