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[africancichlidau]

Yup, like marc says the probe is better fixed securely for two reasons, 1/. the snake probably will climb on it if it gets the opportunity and 2/. most of these probes have a sharpish end to them so if you fix them with the end against the wall of the enclosure the snake can't injure itself on this sharp bit.
If the probe is fixed then silicone should be enough to hold it at the point of entry as long as the capilliary is fixed in such a way that the snake can't get around it. Best way to do this is to drill the entry hole right where you want the probe as then you don't have any cappilliary inside the enclosure to worry about. Hope this helps