What do you use to clean electrode?

Instructions say to use distilled water to clean electrodes

Would this be a good tool for an aquarium?

Sure it will read the Ph for your tank

Does it work well for testing ph in a saltwater pool?

I don’t know anything about testing pool water but it will definitely give you the ph between 4.0-10

My pen has gone way out of calibration. Is there a way to calibrate it against an accurate test kit? I.e. just set the pH?

I second the answer but Alix. I thought it could be recalibrated against the standards they provide and I would be back to using the ph tool. Once it started to give me bogus readings I knew I had a new paper weight. I even bought 7.0 fluid so I could check it before each use so I knew the readings were correct. I have extensive experience calibrating thousands of gauges in a railroad machine shop to NIST and audited by the ARR annually. Knowing I have a good accurate tool is important to me. The quality of this tool is below the mark. I am surprised as I own VIVOSUN products that are more reliable so I thought I would give it a chance. It lasted almost a year for me.