Fish Poo : the ultimate guide.

fish poo from a koi pond
Fish poo starts life as discrete sausage-shaped packages.

Do you want the good news, or the bad? Well the good news is, fish poo should provide all the nutrients your plants require in any aquaponic system. The bad news is, if fish poo is allowed to accumulate it will almost certainly eventually kill your fish. It’s all a matter of balance – once again! Fortunately, it’s a balance that Nature helps us achieve. It just needs a helping hand. That’s what this ultimate guide is all about.

Fish poo really is at the centre of our aquaponic universe. Without it we’d just be left with hydroponics. It’s the stuff that nourishes our plants but could kill our fish. It’s the Jekyll and Hyde of aquaponics. The great thing is it’s being produced on a continuous and steady rate without any work required by ourselves. Our fish are doing all the hard work for us.

How fish poo affects water quality.

Fish basically produce two important types of waste. Inorganic ammonia through the gills and organic fish faeces through the rear end. The former is directly toxic, the latter is not. However, the indirect effects of the accumulations of fish poo can be equally devastating. These effects include;

  • A decrease in dissolved oxygen
  • An increase in dissolved wastes, most notably ammonia
  • A decrease in water clarity
  • Accelerated fish gill clogging
  • An increase in mucus production by the fish in an attempt to protect their delicate gill membranes
  • Encouragement of more aggressive heterotrophic bacteria at the expense of the ammonia oxidising bacteria (AOB)

Several of the above will combine and consequently compound the problems of waste accumulation. The ability of the biofilter to detoxify ammonia is reduced at the same time as there is an increase in ammonia production. The more aggressive heterotrophic bacteria will always out-compete the AOB as they can reproduce much, much faster.

The excess production of mucus will decrease the fish’s ability to absorb oxygen. This also results in greater susceptibility to fish gill diseases. Bacterial gill disease and gill flukes love to live in this mucous.

Fish poo is a major contributor to the total suspended solids (TSS) of a system. It is not the only one however, as algae, dead plant material, dead filter bacteria and uneaten food can all be significant. TSS are solids that are able to be physically trapped by a filter. Specifically, they are solids bigger than 2 microns. Dissolved solids are smaller than 2 microns. They represent a wide variety of materials both organic and inorganic. How they are dealt with has a major influence on the carrying capacity of a fish system. The higher the stocking density of the fish the greater the need to remove fish poo in an effective and timely manner.

Measuring Total Suspended Solids.(TSS)

As far as measuring TSS levels go there are no chemical test kits available. It is a physical process of filtering a known volume of water through a laboratory filter paper, the drying in an oven under standard conditions and weighing the paper and solids collected. Although essentially a simple technique it does require laboratory equipment and is beyond the needs of the amateur. A practical alternative could be to allow a water sample to settle in a glass cone and note the volume of settled solids after 30 minutes, rather than the weight. This is only an approximation as not all TSS will actually settle out. Unless you are experiencing problems with TSS and need to assess the performance of different removal techniques I would suggest that measurements are rarely necessary. Visual observation is usually perfectly adequate.

Water clarity

TSS are a significant factor in determining water clarity. The greater the TSS the lower the water clarity. Many pond and koi keepers love to keep water clarity as high as possible. Unfortunately this is often not the best for the fish. It is perfectly possible for fish to suffer from sunburn if exposed to too much sunlight. Also, they tend to feel much more exposed and vulnerable to predators in clear water, leading to an increase in stress. It is much better to provide some shade and shelter at all times. It was very noticeable that the coarse fish in several Anglian Water’s reservoirs would concentrate close to the inlets where the influx of nutrients would encourage poor water clarity due to algal blooms. This afforded them some protection from pike and fish-eating birds. A carp’s mouth anatomy is clear evidence that these fish have evolved to feed in soft mud on the bottom of a pond, relying on chemical receptors in the barbels rather than eyesight. Contrast this arrangement with the trout that hunts by sight in clear fast running streams.

Water clarity can be measured optically using units of NTU or Nephelometric Turbity Units. No standards are recommended for fish. The legal limit for drinking water in the UK is 4 NTU. Suffice to say that tap water generally is very clear. My own supply is <0.1 NTU so any concerns that may occur in a pond can be addressed, at least in the short term, by simple water changes.

Quantities of fish poo produced.

When estimating fish oxygen consumption and ammonia production rates it is logical to relate these to the food intake. After all, it is the feed that is the ultimate source of the ammonia and it is the food that needs to be ‘burnt’ by the oxygen. This is true also when estimating how much fish poo will be produced. Approximately 40% of the fish food given is excreted as fish poo. Fish food is normally less than 10% water whereas fish poo is over 90%. Speaking as someone who once manually cleaned over 12 tonnes of fish poo from a biofilter I can vouch for the fact that the total volume of poo produced is surprisingly high. (The smell is also extremely persistent)

The exact quantity and quality of poo produced will also vary with feed composition and ration size. Some commercial feed manufacturers now produce diets specifically for filtered aquaculture systems. They are well worth considering as filtration systems certainly seem to benefit from such formulation. I personally have always preferred an expanded, floating pellet.

Water quality problems caused by fish poo.

Although TSS cause obvious visual problems it is their indirect effect on the oxygen and ammonia levels that is far more important. In fact, the effective removal, or breakdown, of TSS is probably the single most important function a filter or grow bed has to perform. More so even than ammonia removal. Nitrification will be seuverely curtailed if the TSS is allowed to accumulate.

Over the years this fact has become more and more recognised by fish farmers and koi keepers alike. Generally, it is better for fish farmers to continuously, and totally, remove TSS from the entire system and not allow it to break it down. Effective TSS removal systems have become of greater importance as we better understand the process of nitrification and fish production levels have intensified.

Aquaponics, on the other hand, relies on the breakdown of fish poo to nourish the plants. Simple removal is not an option. Nitrification inevitably results in the release of additional nutrients. Of course, ammonia is also toxic to fish and its break down is essential for fish health. How this is achieved varies according to the aquaponic system employed.

Grow beds

This is the most straight forward approach and combines ammonia and TSS breakdown in the same container. This is possible as different bacteria occupy different zones within the grow bed. The first zone is occupied by the more aggressive species that thrive on the fish poo. Ammonia oxidising bacteria (AOB) are then able to grow and convert this toxic compound into nitrite. A second group of bacteria can then convert this nitrite into nitrate.

The oxygen demand of fish poo is surprisingly high. In fact, it takes more oxygen to fully breakdown the fish poo than the fish directly consume themselves. This is why it is essential to ensure that the grow bed has an adequate supply of oxygen. If this doesn’t happen the grow bed will go anaerobic and produce toxic compounds including hydrogen sulphide and methane. Nitrification certainly needs oxygen but the fish poo requires significantly more.

Deep Water Culture

This system does not usually have sufficient artificial surface area for the bacteria to grow on and develop effective zones. It relies, instead, on the surface area provided by the growing roots. Effective fish poo removal is essential if the roots are not to become clogged with a heterotrophic bacterial slime. A separate TSS removal system is more easily cleaned and consequently often incorporated. Of course, it is perfectly possible to combine DWC with a media filled grow bed and there is much to recommend this approach.

Fish poo characteristics

Fish poo usually takes the form of discrete, sausage- shaped packages surrounded by a delicate mucus membrane. Sounds good enough to eat -at least to a bacterium. They have a density slightly higher than water and consequently will settle to the bottom if the flow rate is relatively slow. The membrane can easily be disrupted releasing the contents into the general water column. This is not good news for the fish keeper as removal is much harder if a general ‘soup of poo’ is generated. Lesson number one, drain the water out of the bottom of the fish tanks. Lesson number two, allow the water to flow into the grow bed by gravity rather than through the impeller of a pump. This will also reduce pump blockages and maintain maximum water flows.

Effect of diet

All lovers of curry appreciate ‘what goes in’ effects ‘what comes out’.Some diets are better than others. A floating pellet generates less waste. Several of the larger feed manufacturers recognise this and now market feeds specifically targeted to RAS.

General recommendations for aquaponics

  • Keep feed rates well within the carrying capacity of the system. If in doubt always reduce the feed level.
  • Grow beds are usually the preferred option for beginners as they combine the removal of TSS and ammonia in one container. This however is a compromise solution as these two processes need to be separate in a high capacity system.
  • Grow beds should have a depth of 30cms to allow for effective zonation of AOB and heterotrophic bacteria
  • If fish production is of major importance consider the installation of an additional TSS removal system. Vortex chambers and rotary drum filters (microstrainers) are highly effective if rather expensive.
  • Always balance the needs of the fish and plants by managing the amount of fish food given.

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