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hello Ken

thanks for your reply. i am using a multi functional digital tester.
its like a test pen with a digital screen read out, just got it
recently. I dipped the tip of the tester into the water. its something
like this

What is it actually measuring though?
Its probably not measuring any leakage at all.
(It only appears to have one connection so its using induction of some sort.
It will probably detect it if you hold it near the powerhead outside the
tank.)

The fish will swim around quite happy even with a mains extension in the
tank,
I know that from experience.
It does tingle when you reach in to get it but its not painful.
You will probably remember to turn the mains off next time.
I certainly remember now and it happened 38 years ago.