hardness levels and pH, and use acid if appropriate and necessary to maintain the pHwithin the optimum levels indicated above.3.4.4 Filtered waterDepending on the type of filtration (i.e. reverse osmosis or carbon filtering), filteredwater will have most of the metals and ions removed, making the water very safe touse and relatively easy to manipulate. However, like rainwater, deionized water fromreverse osmosis will have low hardness levels and should be buffered.3.5 MANIPULATING pHThere are simple methods to manipulate the pH in aquaponic units. In regions withlimestone or chalk bedrock, the natural water is often hard with high pH. Therefore,periodic acid additions may be necessary to lower the pH. In regions with volcanicbedrock, the natural water will often be soft, with very low alkalinity, indicating aneed to periodically add a base or a carbonate buffer to the water to counteract thenatural acidification of the aquaponic unit. Base and buffer additions are also requiredfor rainfed systems.3.5.1 Lowering pH with acidAquaponic water naturally acidifies because of nitrification and respiration. Withpatience, the pH levels often decrease to the target range.However, adding acid may be necessary if the source water has a high KH and highpH, and there is a high evaporation rate. In these uncommon and exceptional cases,the volume of water to resupply the system is such that it significantly raises the pHabove the optimal ranges and overpowers the natural acidification. Adding acid is alsonecessary if the amount of fish stocked is not sufficient to produce enough dissolvedwastes to drive nitrification and the resulting acidification. In these cases, the resupplyof water will result in a resupply of the buffering agents, carbonates. The natural acidproduction will not be sufficient to react with the buffering agents and subsequentlylower pH. Only add acid if the source water is very hard and basic and if there is notrainwater that can supply the system with KH-free water to help nitrifying bacteria tonaturally lower the pH.
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