The fastest growing water plant, besides Duckweed (that I know of) is Najas AKA Guppy Grass. It sucks nitrates and phosphates out of the water like a sponge and will double in volume every week given warm water and lots of light. I've grown it for years and feed the excess to my tilapia who really enjoy it. I've found that by having a few tanks in a common sump system growing Najas that water changes can be all but eliminated. This is quite handy in indoor recirculating systems when water conservation is an issue. The key is to constantly remove a large portion of the plant each week. The nutrients added by fish food need to be balanced by the removal of plants from the system.

I use this in my fry tanks and crayfish tanks. Najas is normally a big seller on aquabid too.