I have this orp meter for sale used when had pond when o had fish told me when I needed to top potassium up safer than waiting for colour to change you can also measure water helps determine organic load in pond as good as new comes with buffer solution to re caliberate need storage solution for in cap to keep probe moist though there is some in cap with sponge £30 collected or will post £10 extra

4000 gallons queni koi drum and jbr plastics bio chamber drum fed from bottom drain 3 inch gravity skimmer also into drum 55 watt submersible uv 240 litres air and 12 fishI