Gravel bog filters & plants = a clean pond
Deb said the objective is not to kill the algae, but to control it. There are two basic kinds of filters: mechanical filtration (removal of physical debris) and biological filtration (conversion of toxic ammonia and nitrites to nitrates). Gravel bog filters can perform both of these filtration functions, working with Mother Nature to produce clear and healthy water. It works by filtering your pond water through a gravel bed planted with bog plants.
The pea gravel mechanically filters and provides lots of surface area for beneficial bacteria which convert the ammonia & nitrites. The final step or nitrate removal is achieved by the marginal plants which pull out all of the free nutrients basically causing the algae to starve to death.
Some guidelines: A gravel bog filter should be at least 10% –15% of the total surface area of the pond (double that for koi ponds) and no deeper than eight to 12 inches. Plan on moving at least half the volume of the pond through the gravel bed each hour, selecting pumps and pipe sizes accordingly.
Easy to retrofit –gravel bog filters can be built outside the pond, inside the pond or as an island in the middle of the pond. You can even convert your biofalls into a bog filter. First, decide where you want to put it. Using a 2 inch PVC pipe with ¼” holes drilled along the bottom sides of it. Place the pipe work so that you have even distribution of water throughout the bog. It is best to use underlayment on top of the liner to keep from wearing a hole in the liner. It is a good idea to put cleanouts with drain cap at the end of your runs so that it can be cleaned out if necessary. Make sure to check to see if
the water will be distributed evenly before you put the gravel on top of the PVC pipe. It is recommended that you use two pumps; one for the gravel filter and one for the waterfall or other features that you have.
Common mistakes that people make when they build a gravel bog filter are:
1. The bog is too small: For water gardens 10 –15% of surface area should be bog, and for koi ponds there should be 25 –30%.
2. Too deep a bed of gravel - this is the most common mistake made, you need no more than 12” of gravel substrate. If you are adding a Gravel Bog to an existing deep pond area; you can construct a false bottom or shelf using blocks or grating.
3. Wrong size gravel - use 3/8” pea gravel.
4. Top of bog too deep –surface of gravel should beat or above the pond surface area
5. Not enough plants- initially you should plant one plant per foot
6. Wrong plants –there are many aggressive species which can over grow your bog.
