I made an carbon filter. I used an old fresh water filter ( the common ones with the blue
filter pad and carbon) In stead of replacing the filter every 2 weeks I just cut the blue open and emptyd the old carbon. Then I refield /it with some new activated carbon ( Alot more then the filter came with). Then I used some plastic filter matrial (like in a pond filter) to seal the opening. Now I can recharge the carbon while keeping the bacteria in the filter so not to reduce my whanted bacteria. It works great. Also will save me money!