tl;dr; Prioritize the soil ecology and plants (pH 6.4-6.8) over the fish when managing pH. Choose fish species (like Tilapia) that tolerate the pH range optimal for plants and microbes. Don’t overfeed the fish, and keep plants growing actively to maintain balance.

A website purporting to be an information resource requires some semblance of attempting to be information ‘dense’ – therefore, I shall attempt to provide ample ‘density’ without initiating cranial implosions of the would-be readership.

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Prior to addressing pH factors and our recommendations, allow me to briefly identify some basic principles of ecology and iAVs.

An ‘authentic’ / properly functional iAVs operation is an intentional (managed) multi-trophic ecosystem engaging symbiotic mutualism among all cultured species .  The iAVs ecosystem, as also with the entirety of Earth’s biosphere, is literally actuated, ‘driven’, sustained – made possible by – by thousands of complex biochemical processes performed/provided by millions of different microorganism species.  Without the activity of (functions provided by) this vast range of microbial life, all other life on Earth would cease – for vertebrates (you) virtually immediately.  Life sustaining processes (ecosystems) are often referred to by analogy as a “web” , a complex, interconnected, mutually supportive network of interactive life forms and their unique processes. Continuing with the web analogy, the microorganisms are not only the structural backbone of the web of life,  but also the ‘base of support’ that the web matrix is attached to/suspended by.

Therefore, the first priority in an iAVs (and organic gardening, eukaryotic life) is to establish and maintain conditions in support of a diverse, vital soil microbiology.  Of primary influence to the biochemical vitality of any/every organism is the pH (biochemical reactivity – and stability therein) of its unique micro-environment.   pH is defined as the negative logarithm (base 10) of the number of Hydrogen ions in one liter of a solution.  “Most biochemical reactions occur in an aqueous environment” (solution).  Therefore, the pH of an environment’s water – the ecosystem – is of paramount interest in all of biology, including iAVs, at ALL trophic levels, for every organism.

The second priority of an iAVs is establishing and maintaining conditions most favorable to the requirements of the plant life (species) you choose to produce.  The edible fraction of the plant growth represents the dominant portion of the potential revenue stream (economic value) as well as representing the vast majority of the food energy (kcal) output.

Nutrients assimilated by the plants to grow represents a 1:1 removal of the every plant essential element from the soil (filter bed) following the (100’s if not 1000’s of) biochemical transformations facilitated only by the soil biology ‘web’.   In a ‘balanced system’, the elemental composition of the fish ‘wastes’ (products of their metabolism) is assimilated by the plant roots following microbial transformations, and incorporated into plant tissues,  and ultimately removed from the ‘system’ in the form of human food (and non-edible tissue fractions).

In a ‘unbalanced system’, both deficiencies and toxicities can occur, which need be mitigated against in not avoided completely.  Absolute balance of each/every element – between the input (fish feed) composition and the outputs (fish plus plant biomass increase) – is a virtual impossibility.  Some elements may potentially accrue faster than they will be extracted and therefore would progressively accumulate within the sand filter volume over time.

However, when plant growth is consistently maintained at maximal/optimal growth rates, excessive accumulations will not become problematic – but ONLY if (when) the composition of the fish feed input in combination with the rate of feed input (by mass) is approximately balanced with/matched by the rate (by mass) of plant assimilation for each essential element.  Therefore, employ only a ‘well-balanced’ feed formulation and input ONLY as much as necessary to approximate (replace) the uptake rate(s) – mass growth and tissue composition – of the plant production component.

1. keep/maintain the soil ecology thriving, (do not keep flooded (O2 starved) and do not overwhelmed with excessive elemental inputs, particularly the essential metals (Cu, Fe, Mg, Mn, Mo, and Zn).
2. keep the plants actively growing to full extent possible, (sequential planting schedules and crop rotations advised), and
3. feed the fish at rates proportional to the plant growth (uptake rate).  Do not overfeed, do not attempt to maximize fish yield.

For each and every organism in the ecosystem, virtually every biochemical process/transformation is influenced by the pH of its environment.  All microbial processes are influenced by pH, all plant assimilation/growth is strongly influenced by the rhizosphere pH,  and as every aquarists understands, fish health and production is also pH dependent.

Q: IF (when) there are disparities among the optimal pH preference of the various organisms, which aspect/component should be afforded/given preferential consideration?

A: Same priority as above: 1) the soil ecology , 2) the plant(s), and last 3) the fish.

Fortunately, priority 1 and 2 are virtually always identical – or approximately/effectively so. And just as fortunate for our/your purposes, many fish species suited to aquaculture will tolerate, if not also thrive, at pH levels that overlap with the range preferred by vegetable crops (and soil organisms)

The pH level ‘best suited’ to growing-out a particular fish species is often vigorously debated.  Most, if not all, fish species will tolerate a range in pH, certain species much more so than others.  A sub-optimum pH does not imply that the fish suffer in a neurological sense (that I know of),  however, growth rate will typically slow as aqueous pH approaches either the upper or lower limits of a given species preferred range.  When stressed by pH effects, fish can have increased susceptibility and poor response to diseases and parasites. Note that pH is not experienced in isolation with many other water quality parameters (notably Temp. and DO), in combination with pH level, effecting fish health and growth.

With vegetable crops, an optimal pH is somewhat dependent on the species grown and by the type of soil/media in which they are cultivated.  Each plant essential nutrient element is unique in its bio-availably (assimilation) at a given level or range of pH. Specific elements are most readily bioavailable in specific soil/water pH ranges (be that high, low, and/or mid-range ‘gaps’ / ‘windows’)    pH effects on nutrient availability are also dependent on whether grown in a primarily mineral media (aka “dirt”) – or hydro- with water soluble nutrient nuts or in an organic “soil” by/from/with bioavailable sourced (bio-transformed) nutrient forms.

In mineral soils, the elements Copper, Iron, Manganese, Phosphorus and Zinc become increasingly less bioavailable to vascular plants as the pH increases above 7.0.  Regardless of media, metals (except Mo) tend to become increasingly less bioavailable above pH 7.0.   In organic soil, Manganese begins to become increasingly less available starting at pH 5.0 and declining with increasing pH, and increasingly available above 8.0.  Boron and (most importantly) Phosphorus become increasingly unavailable starting at pH 6.0 until 8.0 , above which becomes increasingly available again..  Boron, Manganese and Phosphorus are almost totally unavailable to plants at between pH 7.0 and 8.3,  Other factors (influences) apply, such as CEC, C:N ratio, and the relative tolerance to sub-optimal pH/nutrition of the species grown.

Most vegetable species are tolerant of a range in soil/water pH,  yet none are tolerant of much above pH 6.8. Very tolerant species can accept pH 5.0 to 6.8, Moderately tolerant from pH 5.5 to 6.8 and slightly tolerant from pH 6.0 to 6.8.  Please note, no vegetable crop species does well (not as it could) at pH 7.0 or above.

The pH level ‘best suited’ to growing-out a particular fish species is often vigorously debated.  Most, if not all, fish species will tolerate a range in pH, certain species much more so than others.  A sub-optimum pH does not imply that the fish suffer in a neurological sense (that I know of),  however, growth rate will typically slow as aqueous pH approaches either the upper or lower limits of a given species preferred range.  When stressed by pH effects, fish can have increased susceptibility and poor response to diseases and parasites. Note that pH is not experienced in isolation with many other water quality parameters (notably Temp. and DO), in combination with pH level, effecting fish health and growth.

The pH 6.0 to 7.0 range is also preferred by most if not all microorganisms common to (comprising, forming) organic soils. Yes, pH 7.0 and below does tend to reduce (slow) the Nitrification process somewhat.  OTOH, both the toxicity of un-ionized NH₃ and its concentration as a fraction of the total ammoniacal nitrogen (TAN), increases with increasing pH.   In an aquatic environment, the rate of nitrification declines with pH.  In a soil environment, nitrification is most efficient in well-drained soil (<60% saturated) with high O2 availability, at temperatures between 20 and 30C and the pH near neutral (pH 7 +/- 0.2)

Since the primary product by weight, nutritional value (excluding lettuce), market availability, and economic value from an iAVs operation is the vegetable crop(s), it is recommended that the ‘system’ pH be maintained well below 7.0, with pH 6.4 to 6.8 being a general range acceptable for adequate mineral nutrition of every essential element in virtually all vegetable crops.  Therefore, it is suggested to grow (choose, select) a fish species that either prefers or is generally/largely tolerant of this pH range.

Given the  massive specific surface area of sand available for colonization by nitrifying bacteria, notably in combination with a ‘turbo-charged’ (forced, recharge) availability of Oxygen, nitrification has not been found to be remotely limiting (insufficient) and NH ₃ does not accumulate to levels toxic to fish in iAVs operations.  Also note that aqueous TAN in combination with NO₃ (ratio varies) is the preferred N-source for many vegetable (and other) species.   Ammonium nitrate can and does explode (with sufficient provocation), but can not in water because O2 is limiting.

When Tilapia is the cultivated fish species, pH is note a particular concern since these species have a wide tolerance range.  They can thrive (if slowly acclimated) to as low as pH 5.0.  Tilapia are not only extremely tolerant to a wide-range in pH, but also to every other water quality parameter as well..  Many vegetable species can do well at pH 6.0 and some species down to pH 5.5.

Therefore, all factors considered, it is strongly recommend to maintain aqueous pH consistently, if not also significantly, below pH 7.0, and depending on the cultured fish species’ tolerance level, to below pH 6.8 where possible.     In an iAVs operation, when growing tilapia and tomato (or most other common vegetable garden species), a pH of between 6.4 and 6.6 (+/- 0.2) is ‘basically ideal’ in facilitating adequate nutrition of every plant essential element.

Notes:

Bluegill will grow well down to pH 6.5 and survive in as low as pH 4.0  (other prevailing factors being non-limiting)

Barramundi ‘prefer’ pH 7.0 to 8.5 but are known to acclimate to pH 6.6 (and perhaps lower)