
It is possible to used nitric acid to control pH. This will overcome the problem of phosphate accumulation but nitric acid is a very unpleasant and dangerous liquid. It should never be handled by anyone who is not experienced with aggressive liquids and all good laboratory procedures should be observed. Nitric acid will only solve the phosphate problem, it will not make any difference to the high levels of calcium and other minerals which will eventually accumulate in the tank.
The best solution by far is to use a specific formulation which is usually based on more acidic components. Hard water nutrients were formulated in response to demand from growers in various areas of the United Kingdom such as London, Thames Valley and other areas with very hard water. It was specifically formulated to correct the pH of alkaline water and minimise the amounts of phosphoric acid that are required to maintain it at correct levels. It also takes account of the other minerals to be found in hard water and use of this product will ensure the best possible results in hard water areas.
Another solution would be a reverse osmosis filter (RO unit) which will remove the mineral ions by passing water through a very fine membrane. The water produced can be very pure and will allow normal nutrients to be used with complete confidence.