Do magnetic probe holders cause interference with probes?

[ScubaSteve]

Most of the probes, like pH, are electrochemical probes, so they aren't prone to interference by magnets (or really even AC). A temperature probe will be sensitive to large AC fields (like the 600V field from a T5 ballast) unless it's a grounded tip probe (and if 600V AC is in the tank near your probe, interference is the least of your concerns). If your probe is a magnetic switch of some sort, then you might have problems... But no, magnets should have no effect.