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  1. #1

    Gravity fed filter below water line?

    Hi guys

    I’m new to the forum, and figured I’ll post to get some advice from the experts! I am building a new pond as part of a garden refurb, about 3000litres. The height of the pond will be way above the height of where I plan to put a filter (About 500cm higher).

    According to my research the only solution is to get a pressurised filter?

    Personally I would like to install a bottom drain and do a gravity fed setup (as I am digging anyway) but seems gravity filters need to be at the same level as the water. Does anyone have any suggestions?

    The aim is to not burst the bank and have something easy to maintain. I didn’t feel I wanted the luxury of cleaning a pressurised filter every day

    Here is a link to my design.
    https://1drv.ms/u/s!AnU-ZjoSLb_0gYEpIinet1X0ImMXcg



  2. #2
    Just checking..3000 litres? If so, that’s 650 gallons which is a bit small and wouldn’t warrant a bottom drain imo unless there’s such thing as a small one..2” maybe? Dunno as never looked tbh but you could probably make one easy enough. They’re not intricate feats of engineering
    It’s too small for koi (1 koi/250g??) mind you, I’ve a mate who has small pond and he’s got a couple in there that seem happy enough but they’re only 8” long and won’t ever get any bigger.

    Water finds it own level so yes, in a gravity fed system everything has to line up or it will overflow.

    I think in a pond of that size I’d just use a pump fed system of some kind.
    Probably secondhand, loads on eBay or a small multibay, again, secondhand.

    If you have a few quid you’re really itching to spend then a Draco minima would be awesome but way over the top for effectively a goldfish pond and I think they’re gravity feed only.

    Sorry to piss on your chips mate
    Any size pond is a nice pond though, I love them all.

    Keep us posted though and good luck




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  3. #3
    Oh yea..I think you may have to be careful siting any filter below the level of your pond, if anything breaks or leaks then possibly your pond will empty?

    Experts advise on this?


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  4. #4
    So you miss out on the pleasure of cleaning a pressurised filter every day I’d be looking at a Bead Filter if your having the filter below the level of the pond. 650 gallons is really to small for koi though.

  5. #5
    Very strange, I must have had a bad maths day...

    2.5m across, x 2.5m long x 1.6m deep.

    Is the outer perimeter.

    That’s 10 cubic metres-ish. Considering the trianglar part of the pond, let’s say in total is 8 cubed.

    That’s 8000 litres, 1700ish gallons..

    I’m currently doing some research into bead filters, interestingly a lot of the advice sites omit bead filters from their guides.

  6. #6
    We’ve all done it
    Better size.
    Scrub the pressure filter idea, pretend they don’t exist.

    Redo the link to your design as its broken I think
    Let’s have a looksee


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  7. #7


    Does this help?
    P pond
    M mechanical
    B biological

    If your bottom drain flows faster than your pump (or when your pump fails) then in an open system you’ll empty your pond. Sieves are open so even if you go sieve to bead then you’re asking for trouble unless you can get a sieve up next to your pond at the correct level. I still wouldn’t do it.
    Maybe sieve (at the right level) - > shower on side of pond would be a decent compromise.



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  9. #8
    Senior Member Rank = Gosai Brandlin's Avatar
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    Quote Originally Posted by bigcarpchaser View Post

    Does this help?
    P pond
    M mechanical
    B biological

    If your bottom drain flows faster than your pump (or when your pump fails) then in an open system you’ll empty your pond. Sieves are open so even if you go sieve to bead then you’re asking for trouble unless you can get a sieve up next to your pond at the correct level. I still wouldn’t do it.
    Maybe sieve (at the right level) - > shower on side of pond would be a decent compromise.



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    Big carp chaser is right. But its' not just the bit in bold. If your bottom drain flows slower than your pump you will quickly starve your pump (cavitate it) and ruin the pump. In reality you will NEVER balance the two flow rates.

    IF you REALLLY have to have your filters below your pond, then I would recommend going with the diagram you mark XX. Then add a 'holding tank' with an overflow from your mechanical filter. Anything that would overflow your mechanical filter then flows into the holding tank. In the holding tank you place a float switch with a second large pump. When the holding tank starts to fill it switches on the second pump and pumps the holding tank back into your pond.

    In reality though - depending on the various flow rates of the bottom drain and each pump, your holding tank is going to be a large proportion of your main pond and your second pump will nee to be BIG and it is likely to be running a large portion of the time. Mathematically your first pump running full time plus your second pump runningpart time need to have a much greater flow rate than your bottom drain with no back pressure. And your holding tank will need to be big enough to hold a volume in excess of (the flow rate from the BD minus the first pump flow rate) x the off time of the second pump.

    This is the way certain processes are handled in the chemical industry where significant head is required for the process, but to avoid the cost of pressurised tanks below the feed. HOWEVER its rare for the simple reason there is no fail to safe situation - you still have the situation where a failure of a filter or a pump will drain your pond. Industrially this is handled by having a failsafe valve on the equivalent of the BD where an equipment failure closes the BD valve preventing the pond/head from draining and shuts off the pumps. Its still a real problem if you get a power cut without backup power.

    All in all its NOT a remotely suitable solution for a home pond set up. Gravity is your friend. You want to find a way to work with it in your pond design not work against it.

  10. #9
    No reason why having a bottom drain, into a basket pre filter, pump, bead, UV return wouldn’t work.

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  12. #10
    Yea, bead is “closed” so it’s not gonna overflow.
    Was just pointing out the limiting factors in other methods.



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  13. #11
    Senior Member Rank = Grand Champion andikoi's Avatar
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    juts go bottom drain to nexus,i love my nexus lol,andi

  14. #12
    You can’t site a nexus below a pond only level or above else option 3...boom, flood
    Can’t deny that loads of people love em though, they’re a decent plug and play option if you have the room. They ain’t small are they?
    Good choice secondhand with a Draco chucked in the top I would have thought.


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  15. #13
    Senior Member Rank = Grand Champion andikoi's Avatar
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    kenny are you just digging a hole or having a part sunken and part raised pond, nexus 200 you need 74cm from floor to water level and a 300 is 95cm, so if your going 5ft/1.6m then nexus 200 is half the depth of your pond and would run it easily,if its all sunken then dig down 2 1/2ft at side of pond for filter,its roughly 4ft square(but round lol) andi

  16. #14
    Not sure why my onedrive link doesn’t work. But let’s try drop box...

    https://www.dropbox.com/s/se81gr00fb7jsjx/Pond.JPG?dl=0

    Strange you can’t add attachments to this forum?

    My garden has multiple levels, the pond will be on the upper level. And the filter will be on the lower level.

    I looked at the nexus, it won’t work with my setup unfortunately. I think my option is a bead filter. Looked at instructions for one, looks like I can hook the bottom drain to the filter, and use a pump for the return. Plumb the waste outflow to underground drainage. Done? Looks like there are 2nd hand bead filters for around £200.

  17. #15
    Senior Member Rank = Grand Champion andikoi's Avatar
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    bead filters cant be gravity fed,only pump fed,you would have to have a sieve before the bead or you will be cleaning every day,ive been there and done it,andi

  18. #16


    Does this image work? If so, this is the final construction which I’m hoping to achieve.

    I’d rather not sacrifice the water feature if I can help it. Maybe I could compromise and relocate a filter on the other side of the red bush, though it would be preferable to hide all the gear in the units containing the sink as I have a waste pipe to sewer there.

    Alternatively, instead of all koi, I could resort to getting 1 or 2 koi and stocking goldfish.


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  19. #17
    Senior Member Rank = Gosai Brandlin's Avatar
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    as well as the issues of water level, I think you might be seriously uderestimating the amount of space you require to run filters, clean and maintain them. Measure a few pieces of equipment and look at the complexity of pipework to see how much space it really takes. Not saying you cant make things really compact with a lot of planning, but its not as simple as a pipe to a filter and then back.

    Window is a nice looking detail... how deep is that? deeper it goes, thicker the glass about 1m deep you nee glass in the 20-25mm range. Be interested to know how you intend to join the various glass panels?

  20. #18
    Brandlin..that’s not a window it’s Kenny’s “water feature”
    I’m thinking Salmon not Koi?




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  22. #19
    Though it looks like a window, it’s actually a waterfall. The water falls from the full 2m wall into troughs below like a water wall, then pumped back into the pond, I think I am estimating about 27k litres per hour with this setup.

    Bigcarpchaser did suggest a window there, which would be doable. But maybe it would take the attention from the waterfall. If it was a window, I probably won’t be having it 1m deep, probably a 500mm window with the ability to turn off the trough.

    The waterfall to me seems to be the easier of the 2 set ups and I intend to build a prototype test to see how it would work before the real thing.

  23. #20
    Extreme Koi Member Rank = Supreme Champion Frimley Koi keeper's Avatar
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    Quote Originally Posted by bigcarpchaser View Post
    Brandlin..that’s not a window it’s Kenny’s “water feature”
    I’m thinking Salmon not Koi?

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    Thanks I've just sprayed coffee all over my screen

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