Do you want the good news, or the bad? Well the good news is, fish poo should provide all the nutrients your plants require. The bad news is, if fish poo is allowed to accumulate it will almost certainly 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.
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.
How fish poo effects 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
- An increase in fish gill clogging
- An increase in heterotrophic bacteria at the expense of the ammonia oxidising bacteria (AOB)
Several of the above will combine and consequentlycompound 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.
The excess production of mucus will decrease the fish’s ability to absorb oxygen. This also results in greater susceptibility to fish gill diseases.
Fish poo is a major contributor to the total suspended solids (TSS) of a system. It is not the only one 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.
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.
TSS are a significant factor in determining water clarity. The greater the TSS the lower the water clarity. Some 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.
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 genera lly is very clear. My own supply is <0.1 NTU so any concerns can be addressed by simple water changes.
Quantities of fish poo
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.
Fish poo and water quality
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. If TSS are allowed to accumulate, nitrification will be severely curtailed.
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. Nutrients are released following successful nitrification. 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.
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 surface area for the bacteria to grow on and develop effective zones. 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.
Fish poo and diet
All lovers of curry appreciate ‘what goes in’ effects ‘what comes out’.Some diets are better than others. Generally I have always favoured a floating, expanded pellet as the waste generated appears to be less and more easily removed. 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 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.
- 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.
- Always balance the needs of the fish and plants by managing the amount of fish food given.