On a typically stocked fish aquarium, about 1 cubic gallon of biological media per 20 gallons of aquarium water with a turn over rate of no less than 4 time per hour should be enough. On a fish system, I like to do more than this but I am guilty of major overkill on my filtration systems. I do Reef and planted aquariums differently.
The method you are mentioning of using plants in the sump workr real well too. I have salt water "marsh" like filters that are full of mangroves and macro algaes. I use VHO lighting on the mangrove sump I am running right now. I am building another one for a 265 reef I'm setting up, it will have 55K metal halides on it. I did a fresh water marsh once seveal years ago anf had great success with it. I used mostly bog plants that had thier roots in the mud in the sump but thier leaves were out of the water. I found this to work better than a deep water sump with fully aquatic plants. I did have a bunch of java fern and moss as well as parrots feather. I am building a tank and sump like this over the winter or in the spring time & money permitting.
I don't have a good formula of how much bio media to use when combined with plants, or how many plants to use. My method had been lots of plants and algae with less bio balls. I also run my system on a reverse light program where the sump is on at night an the tank is on during the day. I allway have one of them on and have thier ligh cycle overlap an hour ir two in the morning and evening.
Good luck, have fun with it.....Joel
