up:: Perma MOC
owner:: @Niko

Bog Filters — Pond Filtration

A bog filter moves pond water through a bed of rock and pebble. The media hosts bacteria and beneficial organisms that process fish waste and nutrients, keeping water clean, clear, and healthy; plants set into the media remove extra nutrients and trap fine sediment. Ozponds' position: it's the best filter the average person can build — cheap, simple, low-maintenance.

The fundamentals (true of every design)

Move enough water through enough media. That's the whole job — habitat for bacteria + sediment capture. Everything else is packaging.
Plan the clean-out from day one. Solids accumulate; you need an easy way to flush them. This is the #1 thing that varies by design and the thing people most often skip.
Oversize, don't undersize. "I've got no issues with oversizing a filter — it's much better than undersizing." Oversizing is cheap insurance.
Overflow pipe must be larger than the inflow. Inflow is pump-pressurized; the overflow returns by gravity, so it needs more diameter or it backs up.
Watertight seals: bulkhead fittings for flat-walled containers; uni seals for curved/round containers.

Sizing rules of thumb

Filter volume ≈ 10% of pond volume for a small pond (scales up with fish load).
Pump ≈ 6× the filter volume per hour.
Sizing depends on what you keep (fish/animal type and load). Ozponds has a free calculator at ozponds.com (linked in the video description) with guidelines per stock type. Aquascape contractors instead size wetlands by pond surface area.

The designs (lowest → highest capacity)

1. Mini bog / grow bed — for small ponds (e.g. a wine-barrel pond). Came out of aquaponics (the grow bed is the bog). Water is pumped up into a food-safe plastic container; a central standpipe sets the water level; a wider gravel-guard pipe with drilled holes sleeves the standpipe so media doesn't smother it but water still passes. Pull the standpipe to drain fully; add a hose to divert dirty water to the garden during cleaning. Timber cladding to make it look nicer.

2. Bog-in-a-barrel (any container) — for larger ponds. Water is pumped into the base; a breather pipe prevents back-siphoning if the pump stops. Larger rock (or a void) at the base lets solids accumulate; water percolates up through grading rock/pebble and overflows to a stream/waterfall back to the pond. Add a flush valve at the base. Works great when excavating a pond gives you spoil to build a berm around the barrel — hides it, creates a slope for a waterfall, and lets you "daylight" the clean-out valve for easy back-flushing. Containers seen used: tanks, IBCs, bathtubs — anything with (or able to take) a clean-out valve.

3. Poor man's Aquascape wetland — for larger ponds or where the filter must keep a low profile (no natural-looking berm possible). Same principle: water into the base, a solids-settling zone, easy clean-out. Disperse inflow across the base with a trench liner (or large drilled pipe, or wye the center). A matrix block sits over the dispersal pipe (optional for small fish; use it for larger fish — cheaper DIY alternatives exist). Clean-out via a vertical pipe/port the dispersal pipe connects to (Ozponds used olive barrels slotted onto trench liner, or PVC pipe sized for a pond vac). Bigger filters need multiple clean-out ports / segments so each section drains to its own port.

Other systems mentioned (for follow-up)

Nelson's activated bog filter — same up-flow-through-pebble idea, but they insist pebble be exactly 12″ deep and a specific pea gravel.
David Pagan Butler natural pools — moves water through a regeneration zone using air pumps; used for swimming/"natural organic" pools. Ozponds' open question: how it gets flushed (never seen addressed).
Intake bay / DIY skimmer — Ozponds' own designs; acts somewhat like a down-flow bog but primarily captures surface material + pre-filters the pumps (still provides biological filtration via the pebble). Note: Ozponds is biased against down-flow systems, prefers up-flow.

Application to the La Finka pond

Our build (see Project - Test Pond → main pond, and Design Ref - Pond) is an excavated pond with a planned pump loop + waterfall/stream feature — which maps directly onto Design #2 (bog-in-a-barrel with a berm): the excavation spoil builds the berm, the berm hides the barrel and forms the waterfall slope, and the daylighted flush valve makes cleaning trivial.
The poor man's Aquascape wetland (#3) is the fallback if a natural-looking berm isn't achievable at the chosen site.
Decisions to carry into the build: filter sized to pond volume + stock; pump ≈6× filter volume; overflow pipe larger than inflow; base solids-zone + accessible flush port designed in before digging; uni seals if we use a round barrel.

Source

Ozponds (Kev) — "Bog Filter Diagrams | Designs" (2025). Free sizing calculator + the Nelson's / Aquascape / Butler / up-vs-down-flow / intake-bay follow-up videos are linked in that video's description.