Wet sleeves are a hazard of the hobby
Another hazard of the hobby is a hair falling into the tank, as happened during my first week of tropical fishkeeping. A poor tetra ended up with a strand of my then-long hair stuck in its mouth, which was clearly agitating it, and it was the following day before it finally dislodged after much stress all round for all 6 tetras, my husband and myself, given botched attempts to capture the fish to dislodge the hair. Daft as I might look, I bought a very fetching hairnet specifically for the purpose of water tank changes but still find that a strand of my hair might attach itself to my jumper and inadvertently end up in the fishtank, so I tend to do a thorough check pre- water changes.
Am I being over cautious here in worrying about cleanliness/contamination?
Think it's inevitable that, occasionally, the hands/arm end up in the tank without thinking. However, if you've put on sunscreen or some other type of lotion/cream, then I'd make an extra special effort in such situations to be careful to do a rinse before putting arms/hand in the tank. Yesterday, I had been cleaning, got distracted by a phonecall just as I was finishing, later tried to adjust the filter using a longhandled algae scraper, it slipped and so my hand ended up right inside the tank, then
the realisation dawned on me that I may not have washed my hands post-cleaning. Very luckily, all seems fine so far and I've since done a water change. Situations don't always go according to plan, unfortunately, so best to be prepared.