Very much a newbie question:

I see a lot of talk of monitoring & measuring DO & temperature levels but I can't find tools to measure those.

In a perfect world sensors could be dropped or staked at predefined depths then report back via WiFi with alerts if thresholds are breached. Yup, I'm a nerd.

Any such items to recommend & maybe share a link?

How do you keep the biofilm from growing on it and clogging the O2 sensor? Thermometer would be no problem.

I use a YSI ODO O2/temp gauge to measure those parameters. https://www.ysi.com/parameters/dissolved-oxygen