You could try a Perrys Baby Red. Seen it done a couple times. Bowl or Equisite teacup lotus are another option.
Or... try a red/green tiger lotus. Or Nymphaea Rubra / Stellata / Micranthra / Daubenyana- aquarium lillies can work outdoors too. I have either a red tiger lotus or a Nymphaea daubenyana throwing it's first floating pads in my 20" container. At least for the moment it seems to be the perfect size for the container. No idea if it will bloom or not, but the pads are lovely on their own.
Edit: Forgot about Aponogetons, but those seem to like cooler water and have ugly looms. Fast growers though.
Last edited by Kehy; 07-15-2015 at 06:26 AM.