some easy fast growers to look for
Elodea(aka anacharis), camboba, wisteria, hornwort. get a load of them! plant at least 50% of the substrate. when they get too tall(touch the surface) just pinch off the tops to desired height and replant the tops. might need to get ferts for them depending on lighting...but these plants are easy to look after and grow like mad!
best of luck with your tank troubles. it is really odd?!
cheers