The research team, led by Nathan Howell and Behnam Askarian, built three identical 250-liter aquaponics systems, each combining koi fish with basil grown in deep water culture rafts. The only variable was the source water, drawn from three supplies relevant to the Texas Panhandle: purified reverse osmosis water, fresh groundwater from the Ogallala aquifer, and brackish groundwater from the deeper Dockum aquifer. Each system ran through three sequential basil crop cycles lasting between 32 and 41 days, giving the researchers something rare in the aquaponics literature: temporal replication across multiple growing periods rather than a single snapshot.

The choice of fish was deliberate. Koi are not food fish, but they command high prices as ornamental animals and ship easily in small quantities, making them a practical entry point for a region with almost no aquaculture tradition. Basil, meanwhile, is a high-value herb with established local demand. Together, the pair offered a test of whether aquaponics could work economically in the southern Great Plains using nothing more than the water already beneath farmers’ feet.

On the water side, the numbers were striking. Evapotranspiration across the systems ranged from 0.86 to 1.6 centimeters per day, and overall water use efficiency reached 361 to 615 grams of wet basil biomass per 1,000 liters of water consumed. Daily volume loss hovered around one percent, almost entirely attributable to plant transpiration rather than open-water evaporation. For a semi-arid region where the Ogallala aquifer is steadily declining, these figures suggest that aquaponics could deliver high-value crops with dramatically lower water demand than open-field production.

The fish themselves were remarkably indifferent to water quality. Across all three sources, koi grew at similar rates, with total mass gains of 394 to 435 grams over the full experiment and feed conversion ratios between 1.68 and 1.99. The brackish Dockum system did show a slight numerical lag in later cycles, but the differences were not statistically significant. This finding matters because it suggests that moderately saline groundwater, which is abundant in the region, may be perfectly acceptable for the aquaculture side of the equation even when it stresses the crops.

The basil told a different story. Statistical analysis using mixed-effects ANOVA revealed significant effects of water source, growth cycle, and their interaction on plant dry mass. By the third cycle, electrical conductivity in the Dockum-fed system had climbed above 3 millisiemens per centimeter, well above the roughly 1.5 millisiemens maintained in the fresher systems. Basil yields in the brackish treatment declined accordingly, and the plants showed visible chlorosis. The researchers traced the salinity buildup primarily to accumulating chloride and sodium ions, which neither the fish, the bacteria, nor the plants removed in meaningful quantities.

One of the more counterintuitive findings involved sodium accumulation in the plants themselves. Basil grown in reverse osmosis water, which contained essentially no sodium, accumulated significantly more leaf sodium than basil grown in Ogallala groundwater, which contained 99 milligrams per liter. The researchers speculate that the high calcium and magnesium content of the hard Ogallala water competitively suppressed sodium uptake at the root level, a mechanism they had not directly measured but which is consistent with known ion antagonism in plant physiology.

Iron presented another puzzle. Despite leaf tissue iron concentrations exceeding recommended ranges in all treatments, the basil showed chlorosis consistent with iron deficiency. The explanation lies in partitioning: iron and manganese accumulated preferentially in root tissue rather than shoots, with root iron concentrations reaching 2,450 to 2,880 milligrams per kilogram compared to just 205 to 336 in the leaves. This pattern is characteristic of hydroponic systems operating at elevated pH, where iron forms insoluble precipitates near the root surface and cannot be effectively loaded into the xylem for transport upward. The finding suggests that total tissue iron is a poor indicator of actual iron nutritional status when pH remains chronically above 7.

The mass balance itself proved humbling. The researchers could close the nutrient budget within plus or minus 25 percent for nitrogen, copper, and magnesium, but struggled with most other elements. Phosphorus and potassium always showed negative residuals, meaning more of these nutrients appeared in sinks than could be accounted for by fish feed inputs alone. Sodium was the worst offender, with residuals exceeding 300 percent. The researchers attribute these discrepancies to measurement uncertainty and to unmonitored processes, particularly the possibility of gaseous nitrogen loss through denitrification in low-oxygen zones within the biofilter and sedimentation tanks.

Nutrient use efficiency told its own story. Plants captured only 2.19 to 9.48 percent of the nitrogen and 2.06 to 9.14 percent of the phosphorus supplied in fish feed, values at or below the low end of what previous studies have reported. Potassium uptake was notably higher, ranging from 10.2 to 47.5 percent, but this came with a caveat: leaf potassium concentrations exceeded recommended ranges in all treatments, producing nitrogen-to-potassium ratios far below the published optimum for basil. The researchers suggest that coupled aquaponics systems may structurally tend toward potassium excess relative to nitrogen, since fish feed delivers potassium without any deliberate balancing against crop demand.

Perhaps the most important practical conclusion is that production-scale coupled aquaponics in brackish water regions will require periodic drain events. Because chloride and sodium accumulate relentlessly in a fully closed loop, the only way to prevent salinity stress on crops is to periodically discard water and replace it with fresh supply. This partially undermines the water-saving narrative, but the researchers note that even with such losses, aquaponics would still use far less water than conventional irrigation. They also point out that blending brackish groundwater with fresher sources, or investing in targeted treatment, could extend the intervals between drains.

The study’s multi-cycle design proved essential. Nutrient use efficiency changed significantly over time as systems matured, with the second cycle often showing the highest values before rising salinity in the brackish treatment depressed performance in the third. A single-cycle experiment would have missed these dynamics entirely and potentially led to overly optimistic conclusions about long-term sustainability. The researchers argue that temporal replication should become standard practice in aquaponics mass balance studies, which have historically relied on single crop cycles in the vast majority of cases.

For the Texas Panhandle specifically, the findings offer a qualified green light. Koi and basil can indeed be produced together using local groundwater, including moderately brackish sources, with water use efficiencies that compare favorably to any field-based alternative. The economic model remains untested at commercial scale, and the researchers acknowledge that larger demonstrations will be needed. But the fundamental chemistry works: fish waste can fertilize herbs, hard water can support nitrification, and the region’s declining aquifers can be stretched further by growing high-value crops in closed loops rather than open fields. The caveats about salinity management and nutrient balancing are real, but they are engineering problems rather than fundamental barriers.

Subject of Research: Nutrient mass balance and water source effects in koi-basil coupled aquaponics systems in the Texas Panhandle

Article Title: Nutrient mass balance of koi–basil aquaponics production systems

Article References: Nutrient mass balance of koi–basil aquaponics production systems. (n.d.). https://doi.org/10.3389/faquc.2026.1914289

Image Credits: AI Generated

DOI: 10.3389/faquc.2026.1914289

Keywords: aquaponics, koi, basil, mass balance, nutrient use efficiency, brackish groundwater, Ogallala aquifer, Dockum aquifer, salinity, water use efficiency, Texas Panhandle, sustainable agriculture