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6.5 Alkalinity
Alkalinity is an often-overlooked aspect of water quality but is essential in maintaining a stable system. Alkalinity is a measure of water’s ability to buffer, or resist, changes in pH (Wurts and Durborow 1992). The most common forms of alkalinity are carbonates (CO~3~-) and bicarbonates (HCO~3~-). These carbonates bind to free H^+^ ions, a result of nitrification, preventing a drop in pH. Water with low alkalinity and a steady rate of nitrification experience wide swings in pH, which can be detrimental to the health of fish, plants, and bacteria.
· Kentucky State University6.4 Total Ammonia-Nitrogen
Nitrogen enters the aquaponic system as crude protein in the fish feed. Approximately 30% of protein in the fish food is retained by the fish. Seventy percent is digested and released as solid waste or excreted as ammonia via the gills or as urea (Timmons and Ebeling 2013). Total ammonia nitrogen (TAN) is comprised of two forms that exist in a ratio of un-ionized ammonia (NH~3~, which is toxic to fish) to ionized ammonia (NH~4~+ which in non-toxic).
· Kentucky State University6.3 pH
The pH is a measure of the acidity or basicity of a solution. It is determined by the presence or absence of free hydrogen ions (H^+^), where the more H^+^ present, the more acidic a solution is. An acidic solution has a low pH. The pH is measured on a scale from 1-14, with 7 being neutral. A pH value below 7 indicates a solution is acidic and above 7 indicates a solution is basic.
· Kentucky State University6.2 Temperature
Water temperature is more important in aquaponics than air temperature. Many water chemistry factors are affected by temperature, such as the amount of toxic ammonia (un-ionized) present and the solubility of oxygen. It also directly impacts the health and survival of both fish and plants. Fish are poikilothermic, or cold-blooded. This means that their body temperature is dependent on water temperature. At extreme temperature, fish will stop eating, becoming lethargic and susceptible to disease.
· Kentucky State University6.1 Dissolved Oxygen
Oxygen is required at high levels by fish, plants, and bacteria. Oxygen content is quantified by the dissolved oxygen (DO) in water and is expressed as milligrams per liter (mg/L) (Somerville et al. 2014). The intensive nature of aquaponic systems requires oxygen supplementation. Oxygen can enter the system by agitation at the surface or by diffusers in the water column. Fish stocking density, number and type of plants, amount of organic solids, biological oxygen demand, and temperature are all factors that determine how much DO is needed (Rackocy et al.
· Kentucky State University5.3 Alternative Diets
Alternative diets are a great option to utilize bulk products that are a byproduct of another production system, non-traditional ingredients, or even agriculture scraps. These diets would be prepared on- site and would still be combined in a ratio to meet both the nutrient requirements of the fish and the plant crop. One area where this is seen is in the brewing of craft beer or spirits. The spent grains from the fermentation process (brewer’s grains) typically have a protein content high enough to be used in combination with another protein component, again dependent on the crops to be grown.
· Kentucky State University5.2 Supplemental
A common question among small-scale and hobby aquaponic growers is if they can feed vegetable scraps, insects, or loose grains to their fish. These are known as supplemental diets and only meet part of the nutrient requirement of the fish. This is sometimes seen in traditional aquaculture practices in which fish are contained in large bodies of water where they can scavenge additional foods from the environment. Because aquaponics is a completely closed system, a complete diet must be fed.
· Kentucky State University5.1 Formulated
Formulated feeds are nutritionally complete pellets that are formulated for specific fish and life stage (Figure 15). Unlike other animal crops in agriculture, the nutritional needs of fish vary greatly among species for protein, fat, and carbohydrate inclusions. A carnivorous fish who eats at the top of its food chain, like a largemouth bass, requires a diet with high protein and low carbohydrates. On the other hand, omnivorous or herbivorous fish, like catfish or tilapia, require less protein and can tolerate higher levels of carbohydrate in their diets.
· Kentucky State University4.5 Plants
Stocking and harvesting strategies can also be implemented in the hydroponic portion of the system. The three most common strategies are staggered cropping, batch cropping, and intercropping (Rackocy et al. 2006). Their implementation and success depend on geographic location (tropical or temperate regions), crop variety (leafy vs. fruiting crops), and market demand. Aquaponic producers typically grow leafy green crops, which have a lower value per unit value and high yield. Lettuce, Swiss chard, kale, basil, and other herbs are typically ready for harvest between 3-5 weeks from transplanting (6-8 weeks from seed), resulting in a steady income stream.
· Kentucky State University4.4 Fish Stocking
Fish culture should be well planned, as mismanagement of densities within the system can lead to issues with nutrient build-up/deficiencies, solids accumulation, water quality concerns, and poor fish health. Consider that aquaponic systems typically do not operate with a fish density exceeding 0.5 pounds/gallon. Three of the most common fish production plans are sequential rearing, stock splitting, and multiple rearing units. Sequential Rearing: Sequential rearing involves one tank, containing multiple age-groups of fish (Rackocy et al.
· Kentucky State University