regular hydroponics system where pH increases generally happen towards the upside it is better to use carbonate as a buffering agent than to use citrate or phosphate although phosphate at its regular concentration in hydroponic does provide some buffering against pH moves (without phosphate increases are much more dramatic). For this reason I believe that a phosphate/carbonate buffer seems to be the best choice for most hydroponic growers, taking care to keep the concentrations at levels that do not cause precipitation or phytotoxicity problems.

There is a large number of sites on the net with recommendations on the best buffers in a given region of pH. Generally speaking, the best buffer is buffer whose pK value is in the middle of the desired buffer pH. Just an example: http://microscopy.berkeley.edu/Resources/instruction/buffers.html

Thanks Dr. Herath and Dr. Paul for guiding me and specially thanks to Dr. Kurochkin for providing me this useful link.