Very few people have the enthusiasm to do much with their ponds over winter (us included!) but if you do nothing else, check your kH (carbonate hardness).
It’s the easiest test to do, it’s the cheapest (£6.99) and it’s also an exact test, in that you just count the number of drops it takes to change colour and that is your answer. My kH is 4..couldn’t be simpler, none of this “is it a yellowy green or a bluey green”.
The reason it’s so important, is that your pH will usually drop over winter as a result of a low kH, especially in soft water areas (Birmingham is incredibly soft). We have customers in the Spring who start losing fish unexpectedly. They bring water in, the pH is at 5, kH is at 0 and ammonia is off the chart (because the filter bacteria switch off at a low pH). Even outside of such dramatic occurrences, filter bacteria can not perform their job properly if there are no minerals to speak of in the water, so buffering the kH will stabilise your pH and improve your water quality over all.
It is one of those things that is often forgotten about but is critical to pond maintenance and the consequences can be severe. We have put together an ultimate pond KH keeper pack which includes a kH test kit and a tub of kH buffer which for a single dose would treat 70,000 litres (you are likely to need multiple doses, but it still should keep you going for a bit!).
It’s not a chemical, it’s a mineral so it won’t harm anything!