I started to answer this and then 2) threw me. Can you sketch your proposal and post the sketch on here or email it to me? Include proposed flow rates and pipe sizes and if rigid pipe the number and sharpness of bends. I was going to post my sketched interpretation but I dont have the scanner software installed and it is 11 oclock now.
Are the fall and tank in a manmade or natural hill and if that hill is small please depict its extent on the sketch if possible.
The drain valves on tank can be outside the tank in an easily operatable position.
Can't help on individual tanks or at all on UVs.

you have a long run from the pond to the filter then filter to the fall, the longer the run for the pump the slower it will run, so you need a lot bigger pump to cope with the head pressure although you will only be pumping 700 gallons or so an hour, for such a long run you would want a larger bore pipe back to the fall too, 1.5" at least, 1.25 might cause you a flow problems and overflow in the filter, especialy if the outlet is high in the filter, unless you intend to have two outlets? burried filters are a lot of work in the long run, you have to haul them out of the ground to clean them etc etc.

personaly i dont think any filter with a bulit in UV is up to scratch, there always seems to be problems with them, they need to make them sell so cheaper materials are used which dont stand upto the work load, an inline an independant UV may cost more, but is likely to last a lot longer, and i am in no doudt they are more effective too.

And i doubt theres many box filters that would get away with low maitenance, most need weekly cleaning, even if its oly the small spnges in the top, my own personal choice on filtration is to always have twice the size you need, it might cost a little more but it does actaully reduce the maintenance load, and as Sean says, fitted with slide valves to flush away the crap cuts that work by half, even if you only flush it into a bucket and water the plants in summer, its bettter than having to strip a whole filter down more than once a year.

have you considered making your own filter? its not as hard or as expensive as you might think, you only need spend an hour or so studying how filters work, and seeing how most are designed, and then make one to suit your own needs.

Have you considered a bead filter (low /easy maintenance, mechanical and bio filtration in one) and separate uv unit? Another idea is a small vortex unit 2/3 full of static k1 as your mechanical filtration. Fluidising it with air and pulling the drain valve is the twice weekly cleaning routine. A shower system is another low maintenance option but hard to hide.
Chris.

Sean,
thanks for the reply. See diagram.

The filter would be about 8' from the waterfall.
The ground level probably dips about 3-4" below the level of the waterfall, between the filter and the waterfall.
The pipe would be a few inches below ground level.
The vertical height of the tank is to be determined, that's one of the things I am seeking advice on, but ideally it would be as low as possible.

The Hozelock Titan 3000 pump that came with the filter is working fine, so I could use that unless advised otherwise, I have no problem with its reliability etc. It's base nominal flow rate is 3000 litere per hour. The spec says that would be 2200 with a 1 metre lift, which I think will be about what it will be, or a little less.

So far as water purification is concerned, I am not too worried about having higher flow rates - I never had any problems with green water, nitrites, nitrates etc, so long as I could actually keep the damn Hozelock filter passing anything at all. I am sure that due to the constriction of flow by the Hozelock filter I was probably only circulating the 6500 litres of the pond no more than once every 4 hours or so. This makes me hope that even with longer pipe runs, and the requirement to lift the water an extra 2' or so, I might still get the same or better flow rate with a tank filter.
The current hose from pond pump to filter (not shown) is ordinary 1" flexible crinkle hose. I don't believe the pump outflow will take anything bigger than 1" - though I'd prefer to keep that pipe at 1" anyway, as that will minimise the work I have to do on the pond edge where that hose comes out of the pond.

The waterfall outflow is currently 2 short lengths of 3/4" crinkle hose, connected by a T-connector on the end of the 1" hose from the filter. 2 x 3/4" gives the same capacity, virtually, as 1 x 1", and the waterfall design requires 3/4" hose maximum. I suppose I might be able to rig 3 x 3/4" on a 3-way connector, on the end of a 1.1/4" hose, if I must - tricky piece of fitting though.

So therefore I had hoped to run 1" hose from filter to waterfall.

I think that has answered your questions. When you say the drain valves can be outside the tank - presumably that can't be contrived if the tank is buried, or partially buried.

My DIY Skippy biofilter is working well so far - clear water, no green. NO MAINTENANCE WHATSOEVER! And no UV either, although I have a unit I could attach to the inlet if the filter needs a little help come the Summer.

My pond is about the same volume and fish levels were roughly the same until a heron (I assume) left me with only four 4" fish and two or three 1" babies. I used a 260 litre circular tank to make it. It's about 1 meter diameter, it's half buried, surrounded by rockery, and I grow watercress in the top. Just aerated water pumped down to to the bottom of the filter, where it swirls (it's not a proper vortex though), then upflows through LOADS of fake alfagrog and green pan scrubbers, then flows down my waterfall return.

The only "maintenance" so far, has been to throw some fresh watercress in the top, as the first lot died down during the Winter.

Hmm. 700 gals per hour is just about the 3000 lph rating of my current pump. But with a lift of 2-3', this will reduce to say 500 gals per hour. But see my reply to Sean where I am relatively unconcerned about a somewhat reduced flow rate. Also see why I might find it tricky to fit 1.1/4" hose, never mind 1.1/2" hose.

But the thing you say that really gives me pause for thought is the possibility that the filter will overflow if there is not enough capacity in the outlet pipe. I had reasoned that if the outflow is the capacity of the inflow, then I should not have a problem - is this dumb?

Thanks for the advice on buried filters, I'd kind of thought they might be trouble. Also for the views on separate UV.

OK, so I will have to clean the filters every week then - but presumably this is only to ensure that I am getting some purification? I am right that one of my big advantages with a tank filter will be that even with clogged filter sponges, the flow rate continues, due to the bypass?

I don't really understand the point about slide valves etc. Is the idea that crap gets to the bottom of the tank, and to avoid taking out the sponges and filter medium, I can open a valve at the bottom of the tank and then run garden hose water through the tank to flush out the bottom? This may seem dumb and obvious to you, but with the pressurised filter, there was never any crap that got past the sponges. I should say that the relatively small tanks that I am looking at don't seem like they would be that hard to take out the sponges and media and clean, but maybe I am underestimating that.

Well, there's so much work to do anyway with rerouting hoses (which are buried both within and outside the pond) and electrics, and housing the new filter and concealing it, that I think I'll just pay up and buy a tank, so I think I'll pass on diy for that bit.

One thing to bear in mind is that the idea is that the pond is ornamental - I'm not so keen on fish etc that I am prepared to have my garden filled with green and black plastic and looking like a plumbers yard - so concealement is a high priority, and not that easy.

I hadn't considered a bead filter. Not sure what they are, except I guess the name pretty much describes them. Can you give me a brand to look at?

I've seen one vortex unit on the web - are they basically just tank filters that circulate the water within the tank? So plumbing requirements etc would be the same as for ordinary tank filters?

Sounds good. Not sure I can find anywhere to conceal that sort of tank, which I assume needs to be above waerfall level - though I can see that with watercress it probably is to some extent self-concealing.

Surprised you never have to clean it out though - the pump must pass loads of crap through to it, which I'd have thought must build up with time. Somehow the filter removes all that, does it?

Ah this is what I had imagined
{{gwi:2117020}}

First things first, I suggest you up your pipe size for the supply to the filter and YOU WILL HAVE TO for the GRAVITY feed from the filter to the waterfall. The pump should take up to 40mm hose, NOTE hozelock hoses tails are metric in OD with an imperial 1.5"BSP thread. A lot of hose is still imperial, up to 1" metric=imperial but 32mm=1.26" and 40mm=1.57". Fitting 1.25" hose onto a 32mm is a wee bit of a struggle but 1.5" hose on a 40mm hosetail is a swearing match, it can be done and is best done by getting a piece of 40mm pushfit PIPE, not hose, and filing a 1 in 10 or 1 in 15 taper on one end right down to the bore of the pipe. Heat the hose with a hairdryer, £5 from argos, and twist the hose onto the pipe, the diagram below shows how to twist the pipe and hose so that the helix of the hose opens as they are twisted, if your helix is the other hand than swap the directions of rotation
{{gwi:2117021}}
a little bit of plumbers silicon grease high up the taper will help. Once the hose is well on the pipe let it sit for a while to take a set. Cut of the smaller unused hosetails and file a slight taper on the hozelock hosetail. Take the pipe out using the same twisting action and fit the hose over the hose tail. Alternatively buy an Oase aquamax hose tail as that should fit. I normally use a Hozelock hose tail on an aquamax pump.

Why do I say up your hose size, small bore pipe has a high resistance to flow.
I have a hose "pipe loss" chart presumably form hozelock as it in metric
It quotes the following losses for 25mm/1" hose
1591lph 5 cm per metre of hose
2273lph 10cm per metre of hose
2386lph 11cm per metre of hose????
3409lph 21cm per metre of hose

for 32mm hose 3409lph generates 7cm per metre of hose and 40mm hose generates about 3cm per metre of hose.

I guess you know what follows but I include it for 'completeness'. The pump sees the friction as added lift and that cuts down the flowrat, you are paying for the wasted energy via your electricty bill.

It is up to you whether you up the supply side but the return is a different kettle of fish, the head that pushes water from the filter to the fall is the drop in water level from the surface inside the filter to the surface over the water fall and that is finite. The hose connecting the filter and the fall isnt bothered by whether the flow is pumped or gravity driven, its resistance will be the same as listed above. So, say you have 8 ft of 1" hose and a flow rate of 2273lph you would require the level inside the filter to be 1.6ft, 19.2", above the level over the fall and at a guess more because that will be into the media which its self will add slightly to the required head.
For you information, I am setting the plant pond up with a filter system, that will have gravity feed to a...

Sean,
Thanks very much. Very interesting table of values for losses due to friction. I understand what you are saying.

And I see now that the outflow from the filter presents a different problem (though 3 x 1.5" does seem at first sight to be a tad over the top, that's 5.3 sq.ins of total capacity, versus 0.8 sq.ins from a 1" hose, so 6.5x as much). If I really do need that, then the part right next to the waterfall, which will only accept 3/4" hose, would need 12 (!) pieces of 3/4" hose to give the same capacity as the feed from the filter with 2 x 1.5" hose. This is obviously out of the question.

Thanks for the advice on the imperial/metric problem of fitting hoses. In fact, I have had the devils own job just fitting 25mm hose (presumably) to 1" connectors (presumably) - though these were not the hose tails on the pump/filter, which were presumably also 25mm and somewhat easier so I can't think I would even try to fit 1.5" (=38mm-ish) to 40mm hose tails - I get angry enough with the manufacturers as it is. I have also discovered the benefits of a hair drier - the twist-the-helix technique helps a bit, though eventually I have found the crinkle hose simply breaks at one of the thin bits.

Anyway, many thanks - much food for thought. It's possible I cannot fit a tank-filter, given what you and AJC have said about filter outflow pipe diameters, without massive rework of the waterfall. It might be easier to let the fish die. Or bite the bullet and fit another pressurised filter.

About the Skippy filter... yes, the watercress hides the filter media (the filter is designed to be open-topped), but also helps use up nutrients released by the filter bacteria after it eats the pond gunge... before those nutrients can get back in the pond to feed algae. Wateercress is ravenous and grows fast.

And no, you don't clean them! That's the big attraction! The filter has to be much larger than the usual commercial things, to make sure your get enough mechanical filtration, "dwell time" and enough bacteria to eat up the trapped gunge. (water needs to be well oxygenated too, so you get the right sort of bacteria thriving in it - I put a venturi at the input).

But they are easier to bury, because you don't need much access, and they are so simple. Input pipe goes in at the top, water comes back out at the top. Media can be put in net bags for easy removal out of the top, should it ever be necessary.

You don't need the whole filter to be higher than your waterfall, only the TOP needs to be slightly higher than your waterfall return... and some kind of arrangement like Sean's diagram could work well. The filter is an "upflow" type - water is pumped to the bottom of the filter, flows up through the media, then out at the top. Do a web search on "Skippy Filter" to find Skippy's site and you'll see how it works.

When I originally setup my filter (Green Genie 12000) I had 2 solid (solvent joint) pipes of 1.25 inches ID from the filter box. They were buried, about 6 feet long and the drop was about 3 inches over that length. They were open ended so there was no restriction on the flow and coped perfectly well with the water from the filter. The pump in use being an Aquamax 3500 and the lift from the water surface of the pond to the filter inlet was about 15 inches.
After one summer I decided I wanted to see the moving water so converted it to a stream. I am still using the same filter box and pump and have had no problems with them, including the built in UV. I clean the filter about 4 times a year, a good clean in the spring before it gets going and a quick rinse of the sponges the times. I was late doing the spring clean which lead to an overflow but when I worked out when it was last cleaned it was nearly 5 months so not to bad really.

The picture here shows the filter with itÂs pipes showing

http://retropwr.users.btopenworld.com/images/picture044a.jpg

The picture here shows the filter with all but the lid buried

http://retropwr.users.btopenworld.com/images/Picture073a.jpg

Now that all looks a bit more optimistic, especially about the cleaning.

You used solid pipes because they give better flow? Or because they last longer? Or because they resist animal gnawing?

I'll have a look at the Green Genie, none of my local shops happen to stock them, but I've heard of them.

hi chas - i went the pressure route too originally i now have a pair of fishmate filters which work as uv and heavy particle filters with a yamitsu black box for fines. wasnt planned that way just the fishmate filters didnt do the job for me although polshams seem to work? the thing about the black box is it overflows to safe when blocked ie back into the pond. ret when you said overflow is that what you meant? ie back to the pond ? or overflowed to waste? jo o/

Yes I do mean overflow to the pond return - which is what I thought the fishmate tanks do - a local shop has one feeding a large indoor tank, and there is a cutaway in the corner of the (horizontal) sponges, revealing a tube that leads to the pond return - so if the water can't get throught the sponges, it goes down that tube.

Are the fishmate filters tank filters (they do both tanks and pressurised filters). If so that means you have 3 tanks? Wow.

Thanks for the Yamitsu tip, I hadn't heard of them, I've found them on the web now.

I can't find anything called Polsham - is that a brand name?

hi chaz. the fishmate filters are a pair of 15000 pressure filters with uv. i've removed the media (because it just clogs and turns into an anaerobic smelly mess and doesnt work) then these feed either end of a spraybar in the yamitsu. the fishmates take out the heavy c**p and are backwashable, and the yamitsu takes out the fine stuff and does the bio side. i have a couple of airstones in the yamitsu too, it all seems to work quite well if a little more complicated than originally planned. the yamitsu has a pair of "T" returns which fail-safe back to the pond in the event of blockage. polsham is someone who posts on here lol tricia i think. there are some photos on one of the threads, she has a fishmate pressure filter which seems to work ok? mine didnt d:( incidently i got the yamitsu from our local koi man for about £60 i think including return pipe and fittings. hope this helps jo o/

I used the solid pipe as it was cheaper and was in an area that people would stand to look into the pond and would be less likely to get squashed.
The Green Genie also has an overflow back to pond if the sponges get blocked, provided you install it level - but that's another story :) I would like to think that something as simple as this is standard on all the commercial box filters, it's been on all I've seen at the various lfs's.
(P.S. the only reason I got the G.G. was because the lfs was pushing them at the time, but I haven't been disapointed).