Koi

Keeping the water in your pond clean is vital, so you need to have a comprehensive understanding of your filter system. Dave Hulse examines the chemical filtration stage…

There are three principal stages to the water filtration process in fish holding systems: mechanical filtration, where suspended solid particles are removed from the water column; biological filtration, where dissolved toxic waste ammonia is converted to less troublesome nitrate by beneficial bacteria; and the final stage of chemical filtration, where undesirable compounds can be managed by the addition of specific media.

In the Koi pond, by far the most commonly used chemical filtration medium is zeolite, which is employed to assist the biological filter with ammonia management. However, in rare circumstances, phosphate-specific media can be used, along with that aquarium favourite, activated carbon. Let’s take a look at zeolite first.

Elemental exchange
Zeolite is an extremely useful addition to any Koi pond. It can remove toxic ammonia which may accumulate through a process called ion exchange. This involves swapping ammonium (NH4+) for harmless sodium (Na+).

The other huge advantage of zeolite is that it can be recharged by placing it in a solution of strong saltwater (NaCl). Whether you have a recurring ammonia problem in your pond, or you just want peace of mind, zeolite is an excellent choice.

It is extremely useful to Koi keepers, as the zeolite can be placed into a pond to act as a guardian against any possible accumulation of toxic ammonia, due to malfunctioning biological filtration. The zeolite can then be recharged in salt on a regular basis to keep it at 
peak efficiency.

How to use zeolite
Zeolite should be placed into the filtration system. This is where the greatest flow of water is, so the zeolite will have contact with the maximum amount. The zeolite should be placed after the mechanical filtration stage, which means that it should sit in a filter chamber after the mechanical elements (ie brushes, vortices, and so on).

This will prevent the zeolite from becoming too clogged with solid waste, which will drastically lower its effectiveness as a medium.

Whether the zeolite should be in a chamber before the biological stage or after is more debatable. It stands a greater chance of removing more ammonium in a pre-biological position, however, this may hamper the development of the bacterial colonies in the filter bed.

But if the zeolite is placed in a chamber after the biological filtration, it can ‘mop up’ any ammonia not dealt with by the biological filter bacteria.

If you are using zeolite to get rid of a troublesome excess of ammonia already in your pond, the former seems like a better option. If you’re using zeolite as a ‘safeguard’ against any possible burst of ammonia (a belt-and-braces approach), then the latter would be preferable.

Zeolite should be added to the pond at an approximate rate of 2kg per 600 UK gallons of pond water. A rough ammonia removal rate for clinoptilolite (the type of zeolite recommended for Koi keepers) is 9mg of ammonia removed per gram of mineral.

However, the figure will vary depending on the geographical source of the material, which has a huge influence on its composition and therefore ammonium removal capacity. Remember, not all zeolites are the same!

Power up
Recharge your zeolite in salt water every month to ensure it maintains peak efficiency. It is wise to buy double the quantity of zeolite for your pond. This can then be split into two equal portions. One half can be in the filtration system protecting the fish against ammonia build up, and the second can be placed into a container of strong salt water for recharging.

These two can be swapped around on a monthly basis and the zeolite will always perform at peak efficiency. Always ensure that the recharged zeolite is washed well in tap water before placing it back into the pond.

Bag it up
The zeolite should be placed in a mesh bag in the filter system to contain it and enable easy removal at recharging time. Large chunks of zeolite are much easier to manage, as they can be held in a coarse mesh net and will not shed fines into the rest of the filter.

However, the total surface area per unit of volume of large chunks is low, which will reduce the ammonium removal capacity.

A finer grade of zeolite gravel will perform better in terms of ammonium removal, due to a greater surface area being available per unit of volume. But this fine grade is harder to manage and can shed fragments from the fine mesh bags into the filter.

Zeolite is hugely beneficial in show vats as the lack of any biological filtration capabilities means that there is no other way of dealing with toxic ammonia accumulation. If biofilters are employed in your system, then there is a potential risk of disease transmission.

However, placing bags of zeolite into simple air-driven filters will mean that the small quantities of ammonium released by the fasted show fish will not be allowed to accumulate or cause harm to your fish.

Salt water
There is one instance when you should not use zeolite: if you are salting your pond either as a disease treatment or as a preventative for the winter.

Common table salt (sodium chloride) has numerous health benefits for pond fish – a low concentration (up to 3g per litre) has been shown to increase beneficial mucus production, reducing osmotic potential difference between the fish and the water and thereby reducing urine production.

Salt is also toxic to single-celled parasites, can reduce larval survival of dactylogyrid gill flukes, and can occasionally help in the fight against algae. Thus, there are many incentives for the Koi pond keeper to use salt in their pond.

However, if zeolite is employed in the filter system, they will need to think again, as the increase in sodium concentration can upset the ion exchange process. The best-case scenario is that the zeolite’s ammonium removal capacity will be significantly reduced.

The worst case, though, is that bound ammonium will be released back into the water leading. This can challenge the biofilter bacteria and is quite likely to lead to a spike in the toxic ammonia level in the pond, as well as the potentially serious fish welfare problems that are associated with it.

Carbon print
Activated carbon is a widely used chemical filter medium in aquaria, and is excellent at removing organic substances from the water by adsorption into its porous structure. It is usually made from beech wood, which is heated to around 600°C in the absence of oxygen.

Combustible hydrocarbons are gassed off, leaving the remaining carbon to be reheated to 900°C in the presence of oxygen. This is the ‘activation process’, and small pores are created to which dissolved organic matter from the tank or pond water can bind. Only ‘activated’ carbon will have these adsorbing pores – barbecue charcoal, for instance, is of no use!

In aquaria, carbon can remove a range of troublesome compounds from the water, including end products of the breakdown of wastes (such as many phenolic compounds that give the water a brownish-yellow colour), residual disease treatment, and the brown tannins taken out from leaching bogwood.

As we view fish through the aquarium, the presence of these colorants can leave the tank somewhat less than aesthetically appealing – regular use of carbon in the tank will remove these colourants, leaving cleaner and clearer water.

In the pond, however, such a slight taint on the water colour could rarely be detected. Should a pond keeper (perhaps one with an acrylic viewing window into their pond) wish to employ activated carbon as a chemical filter medium, the quantities required would be inexact, but vast nonetheless.

In tanks or ponds, the carbon should only be left in the filter for a period of 2-3 weeks. After this time, the filtration capacity of the carbon will have become exhausted and simply begin to act as a biological filter medium, until it becomes completely clogged with solid waste and a layer of bacteria.

Unlike zeolite, carbon cannot be recharged and re-used, hence it must be regularly replaced if its services continue to be required in the pond.

Easy does it
In summary, carbon is widely used in aquaria as small amounts can have clearly beneficial effects. The vast volumes of water in the pond, and the difficulty of actually observing any effect of the treatment, render activated carbon redundant in the vast majority of Koi ponds.

However, it must also be acknowledged that activated carbon performs a vital role in tap water purifiers, where its porous structure is used to remove undesirable agents, such as chlorine, chloramine and pesticides.

Pre-filters on the purifier hugely extend the life of the carbon block, and these units are an excellent method of making large volumes of water safe for fish keeping.

Green issues
Another form of chemical filtration occasionally employed in ponds involves the use of phosphate-specific media. Phosphate accumulates in ponds due to the decay of uneaten food and fish faeces.

In conjunction with nitrate, phosphate lies at the heart of algae problems in the pond – if there’s too much, the pond will either go green or become a mass of blanket weed.

The exact formulation of phosphate-removing media is variable across brands (some use aluminium species, others use iron); the theory being with both that the media has a strong affinity for phosphate and will thus lower water-borne levels.

With zeolite and carbon, although there is much variance in the quality of the products, the use of each in the filter system is very similar right across the brands. With the phosphate-removing media, it is less easy to generalise.

However, the basic principle of chemical filtration still remains: the media is best placed in the final stage of the filter system. This will prevent the chemical filter medium becoming rapidly clogged with a layer of waste, thus rendering it completely useless!