Thermometer or foe?

[mpmistr]

Recently I placed a thermometer in my holding tank for my froglettes/juvenile frogs, which has caused much bewilderment for them .
I find them often at times staring somewhat perplexed at the foreign object and even swimming up to it, darting right at the base in some kind of attack.
They also at times attempt to bite it, is this behavior normal or going to cause any future harm in some way? ( I know their bites are weak but just to be safe . . .)

I know this problem sounds a little whimsical, but I just want to make sure that my frogs aren't subjected to possible mercury poisoning( or what ever may be in the thermometer), or perhaps distressed by the new strange "threat" in their home.

I don't think many if any aquatic thermometers use mercury still. For some reason my frogs can't seem to leave the damn things alone either, I always find them floating in the tank when I sure would like them to stay suctioned to the side of the tank.. try to get the ones held on with magnets rather than suction cups.