BulkReefSupplyCom ok, thanks for the quick reply. I'm going to try the Neptune Apex Community Forum to see if they have had some experience with calibrating for freshwater. Have a happy turkey day!! Alan

A little late for you but I would say "Yes, absolutely!" If I'm not mistaken you are calibrating the Apex to the probe, not calibrating the probe itself. The Apex does some math based on what solution you tell it you are using to figure out the exact offset in the probe's readings due to various factors including manufacturing variance and age. Then it just takes the reading from the probe, uses the offset it generated and a bit more math, and displays the calibrated PH. If you do not calibrate the Apex for a new probe, the Apex itself is using the wrong values when calculating the reading off the probe, so you are going to get a bad reading. This might not be necessary on a factory reset Apex with a new Probe, but I would still highly recommend it for the most accurate readings.

Yes, definitely. The double junction probes hold calibration longer, but it's generally assumed all of these probes require calibration over time as liquid can and will eventually seep into the probe and mix with the internal electrolyte used by the probe. As this happens, your probe will give more and more incorrect readings. This is usually okay - a quick calibration and the Apex will use some extra math to determine the correct PH value. Despite the probe's reading being off it is still accurate in the sense that it detects the increase and decrease of PH accurately, it is just using the wrong "starting point", so to speak, to base it's readings. By running the calibration and telling the Apex what PH the probe SHOULD be reading, the Apex is able to accurately account for the degradation in the probe and give the correct PH readings. Eventually the single junction probes may not have enough reference electrolyte (relative to the liquid the probe has been submerged in which has seeped inside), and they may up and die, though they are typically rated for several years of fully submerged use, and the double junction ones typically last quite a bit longer before they stop working properly all together, as the liquid has to breach two junctions before contaminating the electrolyte surrounding the reference electrode, though all of these probes which require the probe's electrodes to contact the water will eventually die, and will definitely lose calibration over time.

Any PH Calibration Solution from a reputable manufacturer should work. 7.0 and 10.0 are good solutions to use if you want your tank in that range, and they are easily found online or at the LFS. BRS doesn't seem to carry the Neptune PH solution, but Drs Foster & Smith carries the exact one supplied by Neptune for $2/pack: www.drsfostersmith.com/product/prod_display.cfm?pcatid=26691