Neptune Systems Lab Grade pH ProbeDescription pH Probe Lab Grade Replace your pH probe or add another for another tank, calcium reactor, etc. Buy this probe if you are: buying an Apex Lite (it does not include a pH probe) replacing a worn out pH probe (we recommend you replace your pH probe every 18 months). adding another tank to your system and purchased a PM1 to do so. want to have a back up probe on hand Lab Grade Probes tend to last longer, give more accurate results, stay