As andrew notes, a pH of 8.6 in reef tank water of the proper specific gravity (1.026 at 25 deg C) is edging on the not-believable. Particularly if the alkalinity is in the 9 dKH range.
Typically, pH probes last about a year, and the calibration on a pH probe will last about 2 months. This is, however, in the absence of encrusting organisms. If junction on a pH probe becomes blocked by growth, that will virtually guarantee a false reading, even if the probe is freshly calibrated (either that, or it will be impossible to calibrate). If you don't have a replacement probe, you can attempt to clean off the junction if you have some muriatic acid (HCl) around. Make a 0.1N solution of it, and soak the probe for about 30 minutes, rinse it off, and recalibrate. If you have concentrated muriatic acid from a pool store or home store, a 0.1N solution is approximately a 1:100 dilution of the concentrate.
Also, if you're not used to handling concentrated mineral acids, you must make the dilution outdoors, and using protective clothing and protective eyewear.
