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calcium and potassium bases are alternated to provide essential nutrients not contained in fish food. Due to their high pH (10-11), these bases must be added with caution and in small doses, as to not raise the pH too quickly. Nitrification constantly drives pH down by depleting the water's total alkalinity and release of H+ ions, so consistent monitoring is important. The need to lower pH is typically not an issue for aquaponic producers, due to nitrification. Producers may need to amend their water source, however, by adding hard water or chemicals to increase alkalinity, which stabilizes or increases pH. If the pH of the system is constantly high, even after cycling, the first step is to make sure solids are not accumulating in the system. Solids that accumulate form anaerobic (low or no oxygen) zones. When anaerobic conditions develop, a process called denitrification, where nitrate is converted back into ammonia, occurs. Alkalinity is released during this transformation, which stabilizes the pH.
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 (NH3, which is toxic to fish) to ionized ammonia (NH4+ which in non-toxic). The presence of one form over the other is dependent on pH and temperature. At high pH (basic) and temperature, there is a higher proportion of toxic ammonia. At low pH (acidic) and temperature, ammonia binds to excess H+ ions and becomes the less toxic form, ammonium. Generally, water quality tests will give the TAN value, which encompasses both NH3 and NH4+. The exact value of toxic ammonia can be determined by taking the number that intersects the recorded temperature and pH (Table 7) and multiplying it by the present TAN value (Masser et al. 1999).
Table 7: Fraction of total ammonia in the toxic (un-ionized) form at different pH values and temperatures.
Temperature (°C)
pH 6 8 10 12 14 16 18 20 22 24 26 28 30
7.0 .0013 .0016 .0018 .0022 .0025 .0029 .0034 .0039 .0046 .0062 .0060 .0069 .0080
7.2 .0021 .0025 .0029 .0034 .0040 .0046 .0054 .0062 .0072 .0083 .0096 .0110 .0126
7.4 .0034 .0040 .0046 .0054 .0063 .0073 .0085 .0098 .0114 .0131 .0150 .0173 .0198
7.6 .0053 .0063 .0073 .0086 .0100 .0116 .0134 .0155 .0179 .0206 .0236 .0271 .0310
7.8 .0084 .0099 .0116 .0135 .0157 .0182 .0211 .0244 .0281 .0322 .0370 .0423 .0482
8.0 .0133 .0156 .0182 .0212 .0247 .0286 .0330 .0381 .0438 .0502 .0574 .0654 .0743
8.2 .0210 .0245 .0286 .0332 .0385 .0445 .0514 .0590 .0676 .0772 .0880 .0998 .1129
8.4 .0328 .0383 .0445 .0517 .0597 .0688 .0790 .0904 .1031 .1171 .1326 .1495 .1678
8.6 .0510 .0593 .0688 .0795 .09114 .1048 .1197 .1361 .1541 .1737 .1950 .2178 .2422
8.8 .0785 .0909 .1048 .1204 .1376 .1566 .1773 .1998 .2241 .2500 .2774 .3062 .3362
9.0 .1190 .1368 .1565 .1782 .2018 .2273 .2546 .2836 .3140 .3456 .3783 .4116 .4453
9.2 .1763 .2008 .2273 .2558 .2861 .3180 .5312 .3855 .4204 .4557 .4909 .5258 .5599
9.4 .2533 .2847 .3180 .3526 .3884 .4249 .4618 .4985 .5348 .5702 .6045 .6373 .6685
9.6 .3496 .3868 .4249 .4633 .5016 .5394 .5762 .6117 .6456 .6777 .7078 .7358 .7617
9.8 .4600 .5000 .5394 .5778 .6147 .6499 .6831 .7140 .7428 .7692 .7933 .8153 .8351
10.0 .5745 .6131 .6498 .6844 .7166 .7463 .7735 .7983 .8207 .8408 .8588 .8749 .8892
10.2 .6815 .7152 .7463 .7746 .8003 .8234 .8441 .8625 .8788 .8933 .9060 .9173 .9271
Source: (Masser et al. 1999)