Routine water testing can let you identify potentially dangerous trends and fix them before the fish are harmed.
In an established pond, with reasonable stocking levels, separate mechanical filtration (to gently remove solid waste) and biofiltration (bacterial conversion of ammonia to nitrIte and then nitrAte: requires oxygen and buffers) and no new fish, there are two tests that I would recomend:
#1 - KH (aka carbonate hardness, alkalinity, buffering capacity). A KH above 100 ppm means that the pH should be stable. If the KH drops below 50 ppm, the pH could crash. When the pH crashes, the biofilters stop working. As long as the pH is above 7.2, it just needs to be stable.
#2 - nitrAte (the end product of nitrogen conversion). NitrAte is almost 5-10 times less toxic than ammonia or nitrIte, but chronic elevated nitrAtes depress the immune system. Plants will use up small amounts of nitrAtes, but can't keep up with a well stocked/well fed koi pond. (The more food you add, the more ammonia is produced by the fish, and therefore the more nitrAte is produced by the filter.
The next big question is how to deal with problematic numbers, and before you can answer that you need to do some more testing: aerated source water. (When tap and well water aren't aerated, disolved carbon dioxide can shift pH and KH values.)
Water changes are not over-rated. Topping up a pond to replace evaporation losses is no substitute for water change. Evaporation concentrates minerals, dissolved organics and other solids and nitrAtes. Water changes dilute out the bad stuff (nitrAtes, DOCS, TDS, GH).
If you are lucky, water changes may be all that is needed to replenish the KH/buffers (consumed by the good biobugs when turning ammonia to nitrIte then nitrAte). If the source water has low KH, you will probably need to adjust alkalinity with baking soda (sodium bicarbonate). A target KH of 150-120 ppm is reasonable unless stocking/feeding levels are very high. If the KH has bottomed out, always check for ammonia before doing a water change or adding bicarbonate buffer. (more on that below)
_______________
In a new pond, or during spring re-start-up, it might be wise to also check ammonia and nitrIte.
If ammonia is high, use a simple binder like dry ChlorAm-X and dose according to the level of ammonia plus 20% (the fish will continue to excrete ammonia). Ammonia shifts to a less toxic form (ammonium) at low pH (and low temps). When buffers are depleted, the pH will crash (go acidic) and the biobugs will stop converting ammonia to nitrIte and nitrAte - the good news is that the pH crash means that the increased ammonia is in the less toxic form. However, if you increase the pH without binding the ammonia, it shifts to the toxic form - bad for the fish!!!
If nitrIte is elevated, the good news is that the biofilter is starting to work on the ammonia. However, nitrIte is just as poisonous as ammonia, it is taken up through the gills and reduces the ability of the red blood cells to transport oxygen (it makes them rusty). By increasing the pond salinity to 0.10-0.15% the higher chloride ion levels will block most of the nitrIte uptake accross the gills. It won't reverse existing damage, but it will prevent more damage.
_______________
GH - only matters if you are interested in raising asagi/goromo/goshiki, or want nice soft skin lustre in high end go-sanke. The only way to drop GH is with a water softener or RO unit ($$$$). GH has no effect on KH or pH.
pH - KH is the early warning system for a pH crash. Mind the KH, and the pH will mind its own business. The more folks test their pH, the more tempted they will be to mess with it. Messing with pH tends to wreak havoc on pond stability. Accept what your source water gives you. My koi do fine at 8.4 - really!!
_______________
Kristin and Kay - Does that answer your questions? I only check in here every couple days, but Lawanna knows where to find me
