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/Aquatic Gardens, Design, Construction & Maintenance

In-Pond (Versus Outside) Filtration

by Robert Fenner  
Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner

Most biological ponds are provided with something in the way of augmented filtration. This is generally considered necessary as a consequence of the system being "out of balance" otherwise; overcrowding, too much too often of the wrong foods... and a desire for clarity.

This article attempts to provide a cognizant discussion of pro/con arguments for/against in-pond filtration issues, applications, suggestions for improving (if you can't/won't avoid in-pond installations) operation and maintenance.

What are they?:

In-Pond filtration refers to placement rather then a a particular type of filtration. For the sake of our understanding here we will divide various in-pond filter modes as permanent/removable, canister, cartridge, and gravel, having reverse or "gravity" (top to bottom) flows.

Alternatively, outside or external filtration systems are remoted away from the pond basin(s). Their comparative specific advantages are detailed later.

Yes/No to In-Pond Filters:

NO! If you can place your filter(s) external to the basins, i.e. outside the pond, do it! Not to reveal my personal preference too strongly or early in this piece, but except for a few limited types of systems: itty-bitty ponds with little livestock, no feeding....use outside filters!

In-Pond Filtration Pro Argument:

1) Laziness:

They're easy to acquire, install and operate! Except for large "gravel" media types, small in-pond "stand-alone" filters are drop-in, plug-in, & away you go.

2) Leakiness:

Where is the filter going to leak? Even if they're seeping like the dickens, the water's still going to end up in the system.

3) Cost for Construction & Operation:

Admittedly in-pond as opposed to outside filtration is cheaper to build out and run. Think about this> The structural "walls" of the filter need not be substantial; they're supported on both sides by water and if they leak, eh! Supports, media... see the previous sentence.

Pumping water around either fluid or air powered is definitely less moolah as well. Lest you're jetting water around for falls &/or super-circulation there is little (head) pressure. You're not pumping up above water level so there's not much cost in moving water within the system.

The Con Argument:

1) Functionality:

They don't work! No, that's not fair; they rarely work. Most in-pond filters are just too darn small; like outside types they need to be @10% plus the surface area and/or volume of the total system. Usually they are pathetically puny with way too much flow rate for the amount of filter media used.

2) Maintenance:

Due to size (and their generally top-down flow path), in-pond filters have to be cleaned frequently; and due to placement are a pain in the keester to do so. Either you must enter the pond (Watch out! It's slippery.), scoop up or pull the filter up by the cord, plumbing, rinse the media, and put it all back (what a pain), or shovel in your extra-long gravel vacuum (hope it's gas-powered) to try to clear out that gunky gravel (good guck/luck!).

3) Disaster-proofedness:

When in-pond filters go "bad", become saturated, there's no waiting, they're gone; & how do you know? Well, you don't. They clog quickly (& solidly). The only recourse is to do your maintenance (daily, weekly) very regularly, or possibly keep a close eye on the rapidly diminishing flow-rate that goes with clogging.

At least with remoted systems you have some greater margin of safety due to air/water surface agitation. This activity enhances toxic gas liberation, respiration, pH stabilization, overall homeostasis.

Well, enough of my negativity, onto:

Applications of In-Pond Filters:

Okay, yes, there are some circumstances/coincidences that call for in-pond filters:

1) Tiny Systems: Of a few tens to hundreds of gallons with itsy-bitsy everything else kind of do okay with little in-pond filters (e.g. Supreme's Poolmaster or Tetra's Luft Air pump and filter combination). Just watch your feedings.

2) Absolutely no space: No fall areas to build within, handy garage or other area to remote the filter? Maybe you are stuck with only an in-pond filter scenario. Build it right, next time.

3) No Fish: and very little feeding/fertilization. A pond with lots of plants, few fish, no feeding...

Maintenance/Operation Schedule:

Get one and live by it! At least once a week in warm months you need to vacuum/clean the filter media/element; maybe while you're doing a simultaneous, frequent partial water change. Yeah.

But, you'll be much better off to have built a remoted filter system outside your pond. You won't have to thrash around with your livestock, fall on your rear; nay, nay, nay. Au contraire! With a simple backwash manipulation, you're done!

The (or at least my) Conclusion:

In-pond filters are inappropriate technology for all but the most puny nutrient-deficient systems.

Get a real, back-washable (manipulable), remoted filter system so you can stay out of the livestock's part of the system. Outside filters do cost some more up front and to operate, but the slight extra cost of building and utilizing an external filter is more than made up in enhanced results and efficiency of operation.

Further Reading:

Anon. 1986. Gravity Flow Filter. Practical Koi Keeping, Vol 1. Associated Koi Clubs of America (AKCA).

Aspray, Rick & Bob Fenner, 1983. Ornamental Fish Ponds Filters: Design, Construction and Maintenance. FAMA 6:6.

Cuny, Joseph F., 1986. In Pond Down Flow Filter. Practical Koi Keeping, Vol 1. AKCA.

Mitchell, Sherwin, 1986. In-Pond Gravel Downflow Filter. Practical Koi Keeping, vol 1. AKCA

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner
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