How deep, how large is your pond? That info will help.

2500 gallons, three foot deep at the deep end. Overstocked with koi.

The real question is where is this water feeding the settlement chamber going to come from in the pond?

Mike

From the bottom of the deep end via a retro drain, with plumbing through the side of the pond about a foot down into either the settlement chamber or directly into the first filter tub, probably a 30 gal rubbermaid type. The pump will be behind the filter media in the second tub. The water level in the tubs will be the same as the pond water level.

Okay now that we know that you will be taking dirtier water from the pond the next question is how much water are you going to be moving through this chamber? How large is the surface area of the bottom of the pond? What size pipe are you going to run from the pond to the chamber? What else are you using for filtration? What is the surface area of the top of the pond? How many and what size are the koi in this pond?
Mike

I'm still in the planning stage at this point. Have not settled on a pump, but will probably go between 2500 and 3200 gal per hour. Pumps this size (Pondmaster) take about a 1-1/2 inch hose, so I'll use at least that for the uptake, maybe 2". Filter media will be in divided stages, course prefilter material from the pond store at first, eventually graduating to quilt batting wrapped around egg crate in the first tub. Probably a bag of scrubbies in the second in front of the pump for bio bugs.

No idea on the surface area (nearly flunked math). Too many koi due to spawn, 25 ranging from 6" to 18". That's down from 35. Gave some away, may give more away.

Currently, I'm running two skimmers, a large and a small, one running to a large bio falls, the other running to a homemade trashcan filter with pre filter and bag of scrubbies. Bare liner.

One or even two 2" pipes will not feed 2500-3200 gph by gravity alone without excessive draw-down in the chambers they're feeding. For pipe this size at that flow you will need direct suction to the pump, which will grind up the debris to make settling far more difficult. A 3" gravity feed line would be close to big enough, but you'd see around 3-4" draw down at those flows. A 4" line would supply more than adequate flow and be reasonably self-cleaning.

Standard settling chambers designed to yield 15 minutes or so dwell time remove a large amount of waste. Smaller chambers with vortex action give similar results. If you want to keep it small and cheap, I'd be considering mechanical pre filters such as a chamber full of brushes. Brushes are relatively easy to clean and are very resistant to plugging if you forget to clean them. Before the pump these chambers can remove a large portion of the bio-load and thereby enhance filtration considerably.

I'll have to reduce the size of the pump then. That's why I'm in the planning stages, still. Never made a gravity fed filter before, so these things are what I need to know. Back to the drawing board.

Another question: How much "drag" is there on a pump behind several layers of quilt batting? What would be the maximum sized pump I could use with a three inch pipe without obtaining a significant drop in the tubs?

I'm now looking at the Pondmaster 2400 Mag as a possibility due to low maintenance, longevity, and U.S. made. I currently have a Pondmaster 1500 Mag attached to one of the other devices, but in watching the return pipe on that one it just doesn't seem to have a whole lot of "pull". I want enough pull from the bottom to actually do some cleaning down there.

A 3" pipe will feed 1500-2000 gph with modest draw-down. I'm feeding 2000 gph through matched 3" pipes about 25' long, each with one 45 degree and one 90 degree fitting; the draw-down is around 2". Angle fittings greatly increase draw-down compared to straight or gently curved runs, so their use should be minimized. Solid PVC pipe can be heated and flexed into gentle curves for a custom fit and improved flow. It's not too hard to DIY such bends, I use a gas grille with a crude shroud over it but a heat gun would probably work too.

My main koi pond is about 14' by 25, both ends carefully bowl shaped to encourage debris to go to the bottom drains. 2000 gph is drawn through the 3" lines and the only thing I need to vacuum out are rocks the kids throw in and sometimes a bit of string algae or leaves that can't really get through the small gap into the drain. On a flat bottom I'd expect around 3' radius cleaning around a decent bottom drain on a nearly flat floor with this flow rate. Fish sorting around down there, though, will slowly move things in. An air dome would extend the cleaning radius greatly, if you're into that kind of fancy embellishment.

Quilt batting plugs pretty easily, so I don't know how one could predict its resistance to flow. I do use Matala matting in an upflow gravity filter for an 8500 gallon garden pond and haven't had much problem with plugging there. For coarse filtration of a fish pond in a smaller footprint than a settling chamber I personally prefer the brushes, though, as they are so difficult to plug yet so effective on coarse debris.

If you have room, I'd think of constructing a small pond next to the main pond for use as a settling chamber. At 3-4 feet deep it can be effective for gravity particle removal and planted with water hyacinth or iris which drop their roots down in for added filtration and beauty. No one who looks at my ponds even suspects that I have a settling chamber out in broad daylight. Hyacinth go crazy in the relatively fertile flowing water. You do have to clean the bottom occasionally. Some use solids-handling submersibles; I gravity drain a couple hundred gallons from the bottom once in a while. The bottom needs to slope towards your pump or drain for better solids removal.

You might split the intake flow of a larger external pump between skimmer and bottom drains so you could fine-tune your draw to suit the equipment and conditions. A low-head external pump around 4000 gph uses well under 200 watts. Whatever pump you use, it is important to consider both intake and discharge piping and its resistance to flow. Smooth PVC pipe will flow much better than the corrugated flex pipe one often sees at big box store water garden departments. 1.5" pipe will be very restrictive on pumps over 2000 gph but work fine on little pumps. For 4000 gph low-head pumps, I minimize the 2" pipe runs by either splitting the flow between 2" pipes or upping the pipe to 3" or even 4" on both suction and pressure sides of the pump.

So how much flow can you get without a significant drop in the chamber. First we have to determine how much 3" pipe this water has to flow through converting all fittings to length of pipe. So as an example I will lay out what might be your plumbing. Bottom drain 5 ft of 3" pipe, 3" 45 degree elbow, 3ft 3" pipe, 3" 45 degree elbow, 3" male adapter,and 3" bulkhead. This piping is in the pond. Outside the pond we have a 3" male adapter, 3.5 ft of 3" pipe, 3" male adapter, and 3" bulkhead on chamber. I am going to assume that the bottom drain has zero friction not knowing what drain you will use. The numbers I list here are either the actual length of pipe or the fitting as expressed as length of pipe in the same order as listed above. So we have 5+4+3+4+6.5+3+6.5+3.5+6.5+3=40 ft of 3" pipe. Based on charts that I have made with this length of pipe I will give you the flow rate and the drop in the chamber. At 1015 gph we have a .5" drop, at 1481 gph we have a 1" drop, at 1831 gph we have a 1.5" drop, at 2156 gph we have a 2" drop, and at 2653 gph we have a 3" drop. If we use 4" instead 3" with same 4" fitting we have the following 5+5+3+5+9+4+9+3.5+9+4=56.5 ft of 4" pipe. So we have 1797 gph at .5" drop, at 2640 gph at 1.0" drop, 3256 gph at 1.5" drop, at 3805 gph at 2.0" drop, and 4700 gph at 3.0" drop. I would agree with bbriggs as far as brushes. As far as the batting goes there is no drag on the pump what will happen is the water level where the pump is will drop as the batting gets dirty. At first there will be almost no drop but as the batting gets dirty the level will continue to drop until the pump sucks in air. How long this will take no one can tell you. With the brushes it could take weeks or months.
Mike

This is too complicated and my pond is a bit small for huge pipes that will be exposed. I think I'll just set a pump on the bottom and run it to the filter.

One of these days, I'll have enough money saved to do a REAL bottom drain the right way, but for now I'll just have to make do. I'm not ready for a total draining of the pond and a re-do. Thanks for your help, guys. You probably saved me from making a huge mistake.

I set the pond up three years ago, and in our area the "eco ponds" are very, very popular, so that's what I went with. I took the rocks out two years ago, so I'm gradually correcting my mistakes, but they have been some expensive mistakes!

I wondered if we'd scare you a little; that certainly wasn't my intent. You sound like you'll get the bug to build bigger someday and enjoy doing it. Running this pond as you are is going to make you that much better on the next round, and very few of us nail it the first try.

You can direct suction from a retrofit drain, as you know, and accomplish much of what you're wanting to do on this round of improvement. If you have an epdm liner it could be relatively cheap to penetrate the wall underwater. The piping in the pond can be easily removable for periodic service or modification. Just a thought, perhaps you're well ahead of me.

Not exactly scared, but the conversation was an eye opener. I guess I thought the water would pull into the tubs at the same rate as what was being pumped out, and didn't stop to consider that it might not be that siimple. I can see now where it would require larger piping from the pond to the tub.

My plan for filtration in that area of the pond is not dead, it'll just have to be adjusted a bit.