My other hat is a Koi Helatcare assistant so I get to play ponds a lot. A voltage in a pond is VERY common and its really important that a) all pond circuits are on RCD's and b) you earth the pond (you can buy Stainless earthing probes)
Pond liners, fibreglass etc are all insulators. It needs something to conect to earth to put the charge into the water (thats normally a person sticking their hand in). However small voltages over long periods do cause ulkceration on the fish
Stick your voltmeter positive in the pond and the negative in an earth socket and see what the PD is?
Normal cause of a PD is a flooded pump, or a leak in the UV unit
Check your L/N, L/E and N/E voltages and resistances and then repeat with the pump(s) out of the water. If the pump is flooded then it provides a path even when switched off (via neutral) all pumps are double insulated and many dont have earths. If its a hozelock titan, then you can pretty well be certain thats the problem
Would be suprosed if a high PD killed the fish though, Simple physics says the current flows around the outside of a conductor and a fish would be at the same PD over its whole length. Much mor elikely in this weather would be low O2 or high Ammonia/Nitrite
