With lot's of plant's, and perhaps two of the Hydro IV sponge filter's in 100 gallons, you will have plenty of biological filtration.
Sponge filter's such as the Hydro create a very large surface area for bacteria to colonize .
Carbon although effective for relatively short period of time before it need's replaced, can also provide more surface area for bacteria to colonize if left for any extended period of time in the flow of water.
What you will be lacking is mechanical filtration which helps remove solids and particulates from the water before they can accumulate on the bottom of the tank/pond, but with enough rooted plants,careful stocking/feeding,and weekly or Bi-weekly water change (depending on waste accumulation) i think it could work.
In any event,,I would never unplug filtration be it sponge filter's or other filtration with live fish present.