elodea is perfect! i see you have some already...when it grows too long just trim off the top and replant it...it usually grows fast so youll have all the plants you need in no time!
other good ones are vallis, camboba, and crypts...all easy and good growers (although vallis needs to be trimmed more as the older leaves die off(dont worry, new shoots are growing) and crypts grow slow...but given time look GREAT!!!
cheers for you and your fish