Concrete Pond Construction

Q. What size of Filter?
I intend to build a pond 14ft x 12ft x 6ft deep. My calculations are 14 x 12 x 6 x 6.25 = 6300 gallons.What amount of filtration is reqd for this?

A. The surface area of the filter should be at least one third of the surface area of the pond. In your case 168 sq.ft divided by 3 = 58 sq ft. My own pond is 30% and has never given problems with water quality over nineteen years. A filter cannot be too big but it can cause a lot of problems if it is too small. If you have only a few fish at the beginning then you do not need to fill all the bays at that time with media. With a six foot deep pond you have a very large volume of water.

Q.Size of filter in relation to pond depth.
I was looking at some previous letters of yours and have a question relating to the size of filters.How can it be that if my pond is a certain size and the filters are worked out on surface area of pond size that theoretically the volume of the pond could vary quite dramatically. As you have said the depth isn't deep enough but if I dug out "X" feet more the volume would increase yet not the surface area.

A. The surface area is worked out at a minimum of 30% of surface area based on the idea that the pond for koi should be 5' 6" to 6' 0" deep. When most ponds were about 4' 6" deep then they used to recommend 25% of surface area. If you have a set of vortex chambers as filters then I would go by the gallonage recommended by the manufacturer as they often use calculations specific to their units, and they are often quite a bit smaller than 30%.

Q. Fiter chamber size.
I cannot tell from the photos how big the concrete block built filters are I presumed 4 x 4 x 4ft.

A. Usually 3ft x 3ft x 3ft. The actual dimensions depend on where they have to be placed. In any case they do not need to be more than 3 ft deep.

Q. Why not a single chamber filter?
I understand the principal of multichamber filters with vortex chambers and so on. But I have heard mention of single chamber filters, one of which claims to contain 6 tons of pea shingle. The author of the article I read seams to be much more in favour of the single stage filter than multi stage. What are your view? And is there anything to bear in mind with a single stage filter?

A. I never consider this is a good idea and definitely do not agree with this. In any case who has enough energy to dig out one bay of 6 tons for cleaning, and it is not a good idea to clean the whole of the filter system in one day. If you have three bays then you can clean one at a time spread over several days. In cases where you have more than four bays then we recommend having two sets of three bays running side by side.

Q. How do a calculate the surface area of the filter?

A. The surface area of the filters should be one third the surface area of the pond. If the pond is say 10ft x 5ft, then this is 50sq. ft and so the total surface area of the three bay filter should be 50 divided by 3 = 16.7sq ft. If you are using a vortex chamber then this is usually not included in the calculations.

Q. Using old media in a new filter.
I read that one member brought the media from his old pond with him.This relates to my last letter in a way because I have some flocor & plastic shapes already in my existing filters plus some foam sheets. What if anything could I do with these.

A. We always recommend that the old media is used in a new filter even if this is only a temporary measure for one year. Make sure that the media does not dry out and just transfer over and the bacteria will survive and hopefully breed and increase.
I do not recommend using the foam sheets in the new filter, but again I would throw them into the top of the new filter so that the bacteria can hopefully move on to the new media. Whenever you wash any old media always try to use pond water and not tap water.

Q. Time to seed new filter.
The fact that it can take up to 2 years for Japanese matting to be fully seeded and working properly came as a shock even though (through your pages) I knew that it takes time for the new material to seed.Is it the same time irrespective of media or is this just for matting?

A. Other media does seem to seed quicker, although Japanese matting is usually is up and running within one season, but you still have to be careful with adding new fish, overstocking etc. The first year is always difficult with a new filter, and usually referred to as "new pond syndrome".

Q. Seeding the filter.
Are those small bottles of "filter seed" effective or will you need gallons of it to work?

A. There are now several of these on the market, including sachets. The total amount used to activate the filter is quite reasonable, and should certainly be used on a new filter, or when you have knocked it back when using chemicals in the pond etc.

Q. Feeding fish with a new filter.
Once you have reintroduced your fish should you feed them the absolute minimum?

A.You would keep the food down, but it will depend on time of year, water conditions, etc. You should introduce a few fish at a time unless you are using a lot of the media from your previous pond.

Q. Are undergravel filters any good in koi ponds?

A. Undergravel filters used to be very popular in koi ponds, but there can be so many problems with them in operation including a possibility of a complete wipe out of the fish stock should the pump stop. They really cannot be recommended unless you are a very experienced koi-keeper, and someone is there most of the time.

Q. Design for home-made filter.
I based the filter system construction on the commercial filter bay type, mainly because of my very limited space , this included four two feet dia by two feet six deep circular plastic water tanks joined together by four inch solvent welded tank connectors inch and a half bottom outlets flushable to waste via gate valves on each tank . The first tank was to be the vortex chamber with water entering near the bottom to create the vortex motion, this also incorporates several brushes. The next two tanks contain japanese matting the fourth and final tank is full of brushes.

A. It is unlikely that your first bay will act as a true vortex as they really need to be specifically designed with a cone shape at the bottom, and even then some work better than others depending on dimensions, pipe inlets, and pumping rate. Still with some brushes in there it should work well as a settlement bay.
Why is the last chamber full of brushes. It should be the first bay after the vortex/settlement chamber so that any debris, silt, etc is removed before it goes through the two bays with Japanese matting.

Q. Type of filter media.
In previous ponds I've used just a vortex + brush unit + flocor + foam to filter the water. Nowadays most pictures show just a vortex system with three 6-8" cartridges of blue matting (in a grid pattern). It just doesn't look a lot to me to provide all of that filtration. Am I just assuming more media volume is better and ignoring the advances of technology?

A. Another question where the answer varies depending who you speak to. The Japanese matting is very popular now, but it does take some time to seed. Nearly all the champion fish in the country have that media in their ponds. To get really clear water you need a sand filter after the filter system.
Brushes work very well, but can be very difficult to keep clean especially if you have a lot of blanketweed etc. Many working people find they do not have enough time to clean them, and is the only reason that they remove them. Flocor (or Alfagrog) is fine. I would not use foam in a big filter systemas it needs very regular cleaning.

Q. Cleaning filter media.
My Koi pond is now 1 year old with reasonably high stock levels. It was designed ( see Mikes’s pond on the site), with about 40% spare bio capacity and involves 2 parallel banks of 1 vortex,1 bio with jap matting and the last one of alfagrog ie 6 vessels. Water quality has been good from the start with twice weekly purges of water from the vessel bottom drains removing the waste. In addition I am emptying and cleaning each vortex monthly to remove muck which clings to the sides. I cannot tell if muck is building up in the bio vessels – however the waste water run off does not smell like the vortex waste
The question is when should I deep clean the other 4 bio vessels and media, at what time of the year, how many at one time and any recommendations on method of cleaning – I would rather not fully remove the matting as it will never go back the same way?

A. The Japanese matting should not be removed. It will only need cleaning when visual inspection shows it is needed.
When this is necessary then empty the water out of the chamber and then use a high pressure washer to clean the matting whilst it is still in place. Make sure that you use pond water to wash it out.
It should not require cleaning very often, and we are talking here about years not months.

The Alphagrog will need cleaning, but it is difficult to say how often as every pond is different. It would normally be about twice a year.
As your pond has been running for one year I would suggest you remove all the Alfagrog from one bay and you can then gauge its condition, and give you some idea of how often you should do it.
In future I would suggest that you can decide if you need to remove all the Alfagrog, or if just emptying the bay, leave the drain open, and use a hose to backflush the filter. You will be able to see what dirt comes out of the drain. Again, you should always use pond water.

The advantage of having two seperate filter systems is that you can clean one section out without affecting the other, and then you can do the other one a week or two later.
Filter bays can be cleaned out any time except during the winter months when the fish are not eating as then the filters are also dormant and best left alone. With a heated pond then you can also do it in the winter depending on the pond water temperature.

Q. Aeration essential for Japanese matting.
I have a idea regarding japanese matting filters that I would like your opinion on. You said that for japanese matting to be of use it needs a pump and airstones in the chamber and then outlet to the pond via a venturi. My suggestion is that if the venturi were on the inlet to the Japanese matting and the outlet to the pond were gravity feed the water would be heavily oxygenated proir to reaching the matting and would still hold a reasonable oxygen content when returned to the pond. I personally don't like the bubbles in the pond that a venturi gives and if this system works I wont have to have them any more, which is secondary bonus. I have attached a sketch of what I mean.  

A. If you are building a koi only pond and eventually you will have large fish as they grow quickly in a good pond then you will certainly have to introduce aeration direct into the pond either with a venturi, airstones, bottom drain aeration, or a waterfall. The latter can only be used in the summer.

Q. Sand Filters.
Do you have any experience with sand filters? I know that they are mainly used as a polisher nowadays but I wondered whether one was necessary if the rest of the system is up to scratch. I've been offered an 18" model (used) for £130.00 and wondered whether you felt this to be worth it. Also, can they be used totally out of the pond by pumping the water from the last bay through the sand filter and back into the last bay or should the system be seperate? Any advice would be appreciated.

A. Quite a few koi keepers have sand filters and they use them in addition to the filter as a means of polishing the water. A few others have left space to fit one later if they decide to do so. Most of those that have them consider they are worthwhile.
The normal way to connect them is straight after the main pump. The pump draws from the last filter bay, through the sand filter, and then through the UV lamp and then to the pond via the venturi. The sand filter needs to be after the filter so that you minimise the amount of debris entering the sand filter.
For a pumping rate of 2,000gph the 18 ins sand filter would be too small, and I would suggest you need a 24" model. This size would cause too big a back pressure on the pump. Also you will need a swimming pool pump to run it as it will NOT backflush properly if you do not use one. Even a Sequence pump is not suitable for a sand filter, and the only suitable pump is a swimming pool pump as you require the pressure.
Suggest 1/3HP ITT Marlow pump and the 24" filter to be ideal if you want to go along this route.
If you use a sand filter then it must be backflushed every other day, and when you are working away, or on holiday then a competent person will need to look after it.

Q. General questions on Filters.
I have now had 3000 gallon pond fiberglassed and have dug out for the filter bays.I am having a vortex and three chambers built in blockwork.
I have some questions to ask.

1. Is one 4" feed from the vortex going to supply enough water to the brush chamber. (Check out the drawing).
2. Does the feed to the brushes from the vortex need to go to the bottom of the chamber, if yes does it go above or below the filter grid.
3. I would like to use Kaldnes K1 in the second chamber on to the third chamber with alfargrog and shells,I am thinking of using two 4" pipes to take the water from chamber to chamber,do these need to go to the bottom of the chamber as in question two.
   In the second chamber the water will flow to the bottom of the third chamber,do I need to make a weir in this so the water flows in at the top or can it flow in at the bottom and up the alfagrog take my pump feed from the top .

A. The answer to these questions is as follows:

1. A 4 ins pipe under gravity can supply 2,400 gallons. As your pond is 3,000 gallons and allowing say 1,000 gallons for the vortex and filters then you should be pumping 2,000 gals/hour. One 4 ins pipe should be OK in this case.
2. In all the filter bays the water should enter at the bottom and flow upwards through the media. The inlet water should be below the grids. The grids should be at least 10 inch above the base so that there is free movement of the water and circulation of any air or oxygen that is in the water.
3. With the Kaldnes media you can use holes in the two 4 inch pipes, but it would be better to use plastic or stainless steel netting to ensure maximum flow. You need a large amount of air for the Kaldnes and probably a 40 litre/min pump is required. I would prefer to put the Kaldnes in the last bay so that most of the muck and debris is removed in the previous bays.
   Two 4 ins pipes can be used as transfer pipes between bays, although I prefer a weir as it creates more oxygen into the water.
   Many koi keepers have stopped using brushes now as they take up too much time cleaning especially when you have even small particles of blanket weed in the pond. You could consider using Flocor.

Why are you using oyster shells. These only seem to alter the pH of the water when they are crushed up, and they should be in net bags so they can be removed.

Q. I've been reading some forum pages and I have noticed that a lot of people do not have filters that are 30% the size of the pond. Does the rule really apply?

A. In the South Wales area koi-keepers have been using the 30% rule for some 20 years and we have found it as very successful.
If the pond is only 4ft deep (not recommended) then you could use 25% due to the lower volume of water.
When visiting ponds around the country there are many ponds that are not really clear right down to the bottom drains.
With the smaller filters reported then you need to ask if they have crystal clear water all the year round, any spikes in ammonia or nitrates, and are they using any additional devices such as sand filters or trickle towers. Stocking rates are also important and in my view most ponds are well over stocked.
A filter cannot be too big, but you can have continual problems with small filters.

If you are using Kaldnes in the first bay then this should cope with any ammonia or nitrate so there could then be an argument for a smaller filter.

With Japanese matting in vortex chamber filters then as you are eliminating the corners then the surface area could be calculated as a square instead of a circle. With these types of systems it is better to be guided by the dealer and make sure you are able to see an outdoor pond successfully using these filters in a garden environment. Many koi-keepers using only Japanese matting often also use a sand filter to finally polish the water.