Some of those mini lilies will grow in a bucket. Look for one with pygmaea in the name. Of other plants, number one for me is caltha palustris (marsh marigold), native, pretty flowers in spring and not invasive. Blue/purple iris (laevigata) is good, the native yellow iris is nice but big and rampant. Water forgetmenot (myosotis) is good round the edges, it does go a bit mad but it's easy to thin out. For oxygenating plants hornwort is good, why people have that canadian pond weed is beyond me. Don't put parrot's feather in, it's a bad one.