Gravel in Koi Ponds

One perennial debate is the one about using gravel or rocks on the bottom of a Koi pond, in lieu of an external biofilter. I do *not* consider this to be a viable option for a Koi pond! In a water garden, where fish are not a prime consideration, perhaps this could be a workable idea. But if the fish are the least bit important to you, I'd not do this.

Chris used one event as the basis for posts on two different web boards, at different times. His approach was just different enough in both posts, I decided to post them both, here.

Mike S.

Undergravel filters in Koi Ponds, as posted on the KoiVet web board by Chris Neaves, 12/15/99

Hi to all,

Posts on under-gravel filters lower down on the board interested me.

We have, in sunny South Africa, a large holiday/casino/entertainment development called the Lost City. The developer, Sol Kerzner also developed the Atlantis resort in the Bahamas that was opened this year.

At the South African resort/hotel they have acres and acres of water in the form of artificial rivers, waterways, lakes, waterfalls etc.

One of the large water features directly outside the one hotels has about an acre of water that is about 4ft deep.

The original filtration concept, a number of years ago, was to have a layer of gravel on the floor about 2ft deep. Under this massive gravel bed was a myriad of pipe work. Large pumps sucked the water down through the gravel bed and returned the water over many massive waterfalls.

This worked fine for a few years. However, the water was never clear. They forgot the 600 koi they added are bottom feeders. So the sludge was continually agitated.

Suddenly after a few years , most of the fish died. Panic stations. It did not take a rocket scientist to discover that the gravel bed had become clogged with organics. The organics decomposed adding to the ammonia load. The clogged bed became anaerobic adding to the toxic environment.

Any remaining fish were removed. The short cut solution was to disconnect the pipe work but to save on costs they left the gravel bed in place. The water was then sucked through a series of pipes at the edges which were placed on top of the gravel bed.

As they decided to do the 'scientific' thing, they analyzed the water continuously over the next few weeks. To their amazement they always had an ammonia reading that was in the toxic range of the scale and there were no fish in the pond/water feature.

As was pointed out to them, they had tons and tons of organic clogged gravel which was decomposing. Literally a giant submerged compost heap. There would be an ammonia problem for a long, long time.

One of the principles of designing a pond (with or with out a filter) is to make it easy to maintain. This should be written into law!

Ponds which are not easy to clean become neglected by their owners - then, no matter how complicated the filter system - the biological balance of the pond degenerates to the point where stress is a mayor factor. The fish become sick and no one really knows why.

Under gravel filter beds do not work in koi ponds for the above reasons.

Regards,

Chris Neaves

Re: A case against gravel floors

Posted on the NI web board, 5/14/2001, 2:54 pm

Hi XXXXXX,

Just read your posting with interest.

Gravel on the floor of a pond - don't do it.

A few years ago a massive casino resort (the Lost City/Sun City) about 2 hours from us had slight a problem in their massive pond. The koi died.

They found massive amounts of ammonia in the water. The consultant on the project paid me a visit. Could not understand here the ammonia was coming from. The whole floor was designed as a gravel filter. A pipe grid under the gravel was sucking the water through the bed via massive pumps and it was being pumped to the waterfalls and rivers surrounding the project.

The koi were alive for a few years now all were belly up.

It does not take a rocket scientist to work out the problem. The under gravel filters were about two and a half feet deep. After several years in the African sun the gravel was clogged with organic material. The decomposition of the organic material (algae) was removing oxygen from the water and producing ammonia and hydrogen sulphide.

The advice given was to remove the gravel and build filters in the surrounding smaller ponds (probably about 50,000 liters, [13000 gallons] each). Modify these filters in such a way as to make the filter media shallow - no more than 30cm (one foot)- replace the gravel with 20mm stone (three quarter inch) and raise the stone off the floor so the filter chambers can be easily and regularly flushed to waste.

Three months later another visit from the consultant. Built new filters, smaller than suggested. No fish in the pond/water feature but masses of ammonia. - Why?

Two minutes of questions produced the answer. They had not removed the gravel, just simply cut the pipe grid off. The water was sucked from several new inlets to the massive pumps. As there was probably one and a half acres of water with gravel on the floor they deemed it too large a project so the short cut was taken.

Koi and gravel floors just do not work. Gravel floors collect vast amounts of organic material. Just the algae will literally put ponds of organic material in the gravel bed each week.

Gravel beds in koi ponds are literally submerged decomposing compost heaps.

Chris Neaves

Editor's Note:

Gravel in ponds WITH an external biofilter or gravel in ponds which is regularly cleaned can be regarded differently from the above. More difficult to maintain than efficient external filtration, gravel beds in Koi ponds require annual tear down and this can be upsetting the Koi and their environment. I wrote this on Bed versus Box filtration.