When I went looking for a way to do this, my criteria were that it be easy to update/change and not block my view of the animal, especially in tubs inside an enclosed rack.
So I found these at one of the office stores:
They are the size of return address labels, are erasable, and don't smear since you write on them with a sharpie. The sort of feel like stick on white-board, so you could probably use dry erase to decent effect, but I find that erasing the sharpie works 100%. I haven't tested for >3 months, but I doubt it would be harder after that, and it erases completely.
I use specially this sharpie size and eraser on them:
As for position, I put them on the shelf of the rack rather than on the tub. (Though I'm sure that would work fine too) The blank one was erased after having writing for a few weeks. I also tag the rack specifically with which probe it uses on my thermostat.
And I just stick them on the glass of AP cages where the substrate dam blocks view into the cage anyway.
Not a ton of room on them, but you could easily use 2 if you needed more space to write.
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