Pond Filter Buying Guide

Generally, most ponds should have two filters. First, the filter protecting the pond pump, the pre-filter which is primarily responsible for keeping the pump running smoothly, but has little to no affect on water clarity or the health of the fish. A pre-filter only needs to be cleaned when there is a noticeable reduction (25% or more) in the flow of the water through the pump. If your water is clear and the flow is good you do not need to clean the pre-filter even though it may look clogged. Pre-filters with foam can be cleaned by pulling the foam out and rinsing it in a bucket of pond or well water then reinserting it into the filter. Pre-filters consisting of a simple pump cage only require occasionally removing of excess leaves, debris, etc., that may collect on the outside of the cage. Cleaning should only require 5 minutes, once a week!

The second filter, the main filter for the pond, is typically a bio-filter and is often incorporated in a waterfall or stream. Your bio-filter is responsible for breaking down fish waste (such as ammonia and nitrites), keeping the pond safe for fish and finally, if adequately sized, for keeping the water clear. Bio-filters must run 24 hours a day, when the water temperature warms to 50 degrees and above to be effective! Even a few hours of a power stoppage can cause the “good bacteria” that lives in your bio-filter to start to die. It takes several weeks for the bacteria to replenish itself after a power outage of more than about 8 hours.

Bio-fall style filters are only required to be cleaned once a year in the late fall or early spring when the bacteria living in the filter are dormant. Cleaning is done by first, removing the filter media (usually lava rock in mesh bags) and foam (if used) from the top of the filter. Clean both thoroughly with a hose (dipping bags of lava rock into a large tub of water works well).  Next, clean any debris/sludge that collected  in the bottom of the filter tank, returning the media (lava rock) and foam to the filter in the same order it was originally. Some filters may also have an drainage hose attached to the bottom of the filter, to aid in cleaning.  If your pump hose feeds in from the top of your filter, make sure it runs all the way to the bottom (under the layers of foam and media). Adjust the bags of lava rock so that there are no gaps along the sides of the filter tank for water to pass by without first flowing through the media.

Another option could be a pressurized filter, which is ideal for small and medium ponds. This self contained filter can be placed partially underground adjacent to the pond and still effectively supply water to the pond.  They are easy to conceal and should be easy to clean (especially when they have a back flush valve included).

A backflush or drainage hose can be added to most filters if desired. By occasionally turning your pump off and opening the drainage hose, you can backflush your filter during the season to improve efficiency. Backflushing shouldn’t be necessary however, unless the fish load is high or the filter is undersized for the pond.

After determining what kind of filter you will need, you will then need to consider where you want it to go. Various factors including ease of installation, plumbing and electrical requirements, maintenance, and pond size should be considered in determining your choice of an external or submersible filter.

External or Submersible (Internal) Filtration: Factors to Consider

Submersible filters are:

  • Installed directly in the pond
  • Intended for smaller ponds (typically less than 2,000 gallons)
  • Easier to plumb
  • More difficult to wire for electricity
  • More difficult to access for maintenance
  • Easier to disguise

External filters are:

  • Installed outside of the pond
  • Intended for larger ponds (typically up to 21,000 gallons)
  • Usually don't have to clean as often
  • More difficult to plumb
  • Easier to wire for electricity (and less likely to cause shorts)
  • Easier to access for maintenance
  • Harder to disguise (and often requires housing to protect from the elements)