Under gravel filters: upflow vs downflow?

[combatwombat]

I have this unshakeable fear that the bottom of my 4.5' deep pond with gravel and no bottom drain is going to become a muck magnet that I need to do something about now—while I have a chance to address it during the construction phase. You can thank Koiphen for this obsession I'm having.

To clarify: I am not building a dedicated koi pond, but I would like the option in the future to add a couple larger fish like koi without having to resort to the grueling maintenance of drain and cleans to keep the bottom clear.

One of the most novel ideas I've seen to address this is an under gravel filter. This is NOT a bog filter outside the pond, but it works basically the same: Introduce water flow evenly throughout the bottom of your pond, cover it with a few inches of gravel, and let nitrifying bacteria do the rest.

The idea is that sinking solids will get sucked into the gravel and consumed by the nitrifying bacteria. Obviously wouldn't work for leaves and twigs but for fish poop and what not.

Here's what it what it looks like (pre gravel installation):

There is a ton of info/discussion about this on these two articles on Pond Trade Mag and I've seen Eric Tripplet talk about how he installs these on his hybrid/crossover ponds (i.e. water gardens or fish ponds that also have some koi in them).

It looks like these can be successfully designed as either suction grids (pull water into the grid and dump it somewhere else) or pressure grids (pump water into them like we do with pea gravel bogs).

As a pressure grid, it would work almost exactly the same as a pea gravel bog except that the gravel is thiner (3-6" instead of 12" minimum) and no plants, obviously, since this is at the bottom of the pond.

Questions for discussion:

Why would you choose suction vs. pressure? Or vice versa?

The discussions over this system at Koiphen are basically, "Don't even think about doing either of these, but if we can't talk you out of it, do the pressure grid." Haha.

 

[islandboy]

I chose pressure as a way of keeping the gravel at the deep end clean.

 

[combatwombat]

I chose pressure as a way of keeping the gravel at the deep end clean.

Rock on. There’s so little info out there about this for ponds.

Why did you choose pressure over suction? How long ago was yours installed and how is it working out? Do you have a build thread I can check out?

 

[islandboy]

Based my decision on pond shape and 45 year old memories of aquarium systems. I don't want the gravel to be the septic system. The pond drains into a mini bog I hope to expand during next winters shutdown.

https://www.gardenpondforum.com/attachments/pondop-jpg.139189/

in operation less than a month and appears to be working fine. Though the more I learn the more I want to improve on.

 

[combatwombat]

Cool. Look forward to seeing how you like it as time goes on.

 

[brokensword]

I have this unshakeable fear that the bottom of my 4.5' deep pond with gravel and no bottom drain is going to become a muck magnet that I need to do something about now—while I have a chance to address it during the construction phase. You can thank Koiphen for this obsession I'm having.

To clarify: I am not building a dedicated koi pond, but I would like the option in the future to add a couple larger fish like koi without having to resort to the grueling maintenance of drain and cleans to keep the bottom clear.

One of the most novel ideas I've seen to address this is an under gravel filter. This is NOT a bog filter outside the pond, but it works basically the same: Introduce water flow evenly throughout the bottom of your pond, cover it with a few inches of gravel, and let nitrifying bacteria do the rest.

The idea is that sinking solids will get sucked into the gravel and consumed by the nitrifying bacteria. Obviously wouldn't work for leaves and twigs but for fish poop and what not.

Here's what it what it looks like (pre gravel installation):

View attachment 139403

There is a ton of info/discussion about this on these two articles on Pond Trade Mag and I've seen Eric Tripplet talk about how he installs these on his hybrid/crossover ponds (i.e. water gardens or fish ponds that also have some koi in them).

It looks like these can be successfully designed as either suction grids (pull water into the grid and dump it somewhere else) or pressure grids (pump water into them like we do with pea gravel bogs).

As a pressure grid, it would work almost exactly the same as a pea gravel bog except that the gravel is thiner (3-6" instead of 12" minimum) and no plants, obviously, since this is at the bottom of the pond.

Questions for discussion:

Why would you choose suction vs. pressure? Or vice versa?

The discussions over this system at Koiphen are basically, "Don't even think about doing either of these, but if we can't talk you out of it, do the pressure grid." Haha.

CW, not really an answer to your question(s) but I consider the fact that those at Koiphen deny the viability of upflow wetland filters and demand water changes to successfully keep koi as a deterrent to taking much advice. I've read too many threads there and this narrow mindedness is off putting to me. Doesn't mean they don't have good points, but seriously, they are such gearheads I have to bypass most of those discussions.

Relative to your ideas, I'll weigh in and say unless you overstock your pond with fish, I doubt you'll have any problems at all. @addy1 has quite a few fish and yet the pond bottom is easily clearly seen as uncluttered by fish wasted and debris. I don't think she has a bottom drain, either, though I'm not sure.

You're trying to balance this as a pool for the kids and a place for fish; I think you're going to have a problem getting it to be both. Like a natural pond/lake, the kids can swim in anything you construct as a home for your fish, imo, and be fine.

With all that said, I'll follow your progress and see if either system gains anything for you that can be relayed to others that follow in your steps.

 

[combatwombat]

I consider the fact that those at Koiphen deny the viability of upflow wetland filters and demand water changes to successfully keep koi as a deterrent to taking much advice

I'm with you, @brokensword. I find a lot of the discussion over there to be pretty nauseating. It's always where I end up finding discussions about these less common filtration strategies, though. Those folks love to talk about that stuff—even if it's to shit all over any idea other than a bare liner with a gravity fed bottom drain. Haha.

It's just a different mindset over there. Most of them are trying to cram as many pounds of thousand dollar poop machines into as pristine of water as they can. I can appreciate them for that as it has led them to solutions that achieve and maintain that kind of pond.

You're trying to balance this as a pool for the kids and a place for fish; I think you're going to have a problem getting it to be both. Like a natural pond/lake, the kids can swim in anything you construct as a home for your fish, imo, and be fine.

You're totally right about that. I know that I won't create the perfect swimming space or the perfect garden pond, but hopefully as long as I implement some of the pond community's best practices, I'll get a good compromise that everyone is "happy enough" with.

 

[combatwombat]

I'll weigh in and say unless you overstock your pond with fish, I doubt you'll have any problems at all.

I hope you're right!

I just have a voracious appetite for knowledge, especially when I'm taking on a new challenge and the stakes feel kind of high. That's what drives me to explore and understand all these strategies. I want to remove as many "things I don't know that I don't know" from the equation. When I do something "wrong" I like it to at least be because I consciously made a compromise and not because I was just totally ignorant.

Pretty sure I'm going to end up with a few of those anyway!

 

[combatwombat]

those at Koiphen deny the viability of upflow wetland filters

I think this goes back to the "pristine koi jacuzzi" mindset that is dominant there. I don't feel like they deny that a wetland filter works, but they see it as inferior because their koi-specific filters can do the same amount of filtering in a fraction of the space and are easier to back flush—even if that means they have to do it twice a week instead of once ever 10 years. lol

I will say, though, Koiphen is where I learned about RDFs, and those things are stinkin' cool. I won't have one for this pond, but I could see wanting one in the future.

 

[brokensword]

I think this goes back to the "pristine koi jacuzzi" mindset that is dominant there. I don't feel like they deny that a wetland filter works, but they see it as inferior because their koi-specific filters can do the same amount of filtering in a fraction of the space and are easier to back flush—even if that means they have to do it twice a week instead of once ever 10 years. lol

I will say, though, Koiphen is where I learned about RDFs, and those things are stinkin' cool. I won't have one for this pond, but I could see wanting one in the future.

It's not that I'm against gearheads--I can be one too, easily, but I've read enough comments there and they seriously don't think a bog makes the grade. They're also against having plants (maybe by necessity) and I'm proving them wrong there, too. Not that the koi aren't nibbling but I'm not a purist and the 'fish are everything' isn't my mantra. I like it all. When I open the pond in the spring with my turtles still in the basement overwintering, there's an element missing. That's also why I overwinter some plants--so I get them into the pond earlier. I like the whole system, not just straight sided, bare bottomed, plantless-in-the-pond, ponds. I've seen enough of their videos to think 'damn, my whole pond didn't cost as much as their filter systems'. And like you said, weekly/daily maintenance is fine if you like constantly fiddling (which I think is a large part of this too) and I'm fine doing what is necessary but when I find a system that works just as well or better, AND is less labor, I'm going that way. To dispute the many posts of boggers here that have actually had success seems narrow-minded, as I said. So, I'll peruse Koiphen just for some different takes and surely they know how to medicate/care for fish better than most, but I still find too many of their opinions to be dismissive of any way but theirs. Telle est la vie!

 

[brokensword]

I hope you're right!

I just have a voracious appetite for knowledge, especially when I'm taking on a new challenge and the stakes feel kind of high. That's what drives me to explore and understand all these strategies. I want to remove as many "things I don't know that I don't know" from the equation. When I do something "wrong" I like it to at least be because I consciously made a compromise and not because I was just totally ignorant.

Pretty sure I'm going to end up with a few of those anyway!

Oh, I approve of your appetite for knowledge; no issues, mate! I'm more in the KISS camp as I always look at it like this; the more complex it is, the more issue when it comes time to fix it. And since I'm the one always having to fix it...built my whole house with this concept in mind, hence shallow pitch on the one-story roof!

 

[Jhn]

If it were me and I was to do it, (thought about it but nixed it) I would go with the upflow because I wouldn’t want to deal with it when it eventually clogs doing it down flow, which common sense wise if I have something pulling water down to the pond bottom it is going to pull stuff down there that I really don’t want down there or stuff that won't break down quickly. Like @brokensword said prefer to keep my set up simple, easy to fix things when something breaks down or needs tending to.

I don’t really see it as necessary though other than just something fun to try in the pond to see how it works. It’s how new and better ideas are implemented.

My current pond now goes both ways the 11 year old part of the pond is mostly bare liner part rock/gravel sand where the plants are and has a magnolia tree next to it.No bottom drain and zero crap accumulation in the deep end other than pea gravel from the koi picking it up and spitting there. Couple magnolia leaves here and there on the bottom but most get caught by the skimmer at the end of the pond. The new part of the pond has large boulders and a shallow layer of 3/4 gravel on the bottom. We’ll see how it does, but between the 2 bogs and the way the power heads move water around the pond to the intake bay not concerned with crud accumulation.

 

[combatwombat]

@Jhn: That line of thought makes sense. The main reason I'd be interested in a suction grid is because it would mean I get to send more water to another waterfall!

 

[brokensword]

@Jhn: That line of thought makes sense. The main reason I'd be interested in a suction grid is because it would mean I get to send more water to another waterfall!

ha, you make me laugh, CW; I'm SURE I read, somewhere in this GB-length thread, that you were going to eschew a waterfall in this build! Did I imagine that?????

 

[twoheadsbrewing]

Are you also going to have a skimmer?