Judy, KH is a measure of the alkalinity or buffering capacity of your pond water. The higher the value the better the pond is able to resist swings in pH. It is sometimes called "carbonate hardness" but actually has nothing to do with hardness - that's just terminology left over from water treatment plants. You should have your alkalinity (KH) in the vicinity of 80 mg/l and preferably a bit higher to help your pond avoid what is typically termed a "pH crash" which can easily result in killing your fish. Fortunately it is a very easy thing to adjust. Go to your grocery store or big box store (Costco, etc.) and buy a very large bag of baking soda - NOT baking powder. For your size pond you are going to want to add approximately 2 to 3 pounds of baking soda. The nice thing about all this is that if you add too much not to worry...no impact on your fish or your plants (unless you suddenly dumped in 20 pounds!!! LOL). Given that your pH is already sitting very close to 8.3 you don't need to worry about adding it and having much of an impact on anything. Below is a URL connection to a chemical addition calculator/spreadsheet in Excel, which on the third tab on the bottom of the spreadsheet will allow you to see the impact of what happens when you add sodium bicarbonate. Just enter your pond volume in gallons and play around with the amount of sod. bicarb you might add...you'll then see the impact on your alkalinity. If you are at all concerned about all of this then add say a half pound or so or a couple of cups around the pond, give it a few hours to dissolve and mix and then check your KH. Once you get the KH up there where it needs to be you'll find that it should remain stable for several weeks at a time. Good luck and enjoy the pond!
---David

Here is a link that might be useful: clicking on this will begin the download for the Excel file

Just as side info, your PH and hardness may drop/change as the pond ages. It may also change if you get heavy rainfall that fills up the pond when you usually use tap water or if you only use rain water to top the pond after initially filling it with tap water. The hardness can also drop if you have rocks that were leeching and have stopped. Concrete can harden the water for a year if it is not properly cured, if you have bricks or anything. The previous post gives excellent info but you may want to watch the parameters for a while before you do anything, it may fix itself if you normally have different readings.Hardness is usually pretty stable and self corrects to its normal reading.If you used tap water last year and this year it is also possible that the city changed the additives to the water affecting the hardness which would explain the change, you may have to adjust the hardness after every water change. Also your test kit chemicals may be aging and that can sometimes affect the readings, but if it remains low or you see PH swings than definitely follow David's advice

I believe a KH range between 7 to 10 degrees or so is desired, about 125 to 180 ppm.

Thanks everyone. I did end up putting in 25 tsp of baking soda (based on formula of 1 tsp / 100 Liters to raise it 2 deg dH) to see if it went up 1. However when I rechecked the PH it was almost at 9.5.
The PH test kit has expired. I went to Big Al's today and bought a new one, only to return it after dinner as it was for a small PH range (5.2-7.6). I'll have to get a new one at the pond shop this weekend. In the meantime, I retest with the old one. The PH reading was 9.0 (had heavy rains last night). When I retested the KH.. it hit me that I did it incorrectly... counting the number of drops that it took from yellow to blue... instead of going back to yellow again. Now done properly its 4 drops which is what is was last year. Whew! Maybe it could be higher, but I feel much safer with the reading of 4.

David, thank you for the worksheet. For the baking soda section, I thought it would give me a PH reading, like 7.5, etc. instead it gives me reading of 28 mg/L as CaCO3 for 1 pound of baking soda. How does that translate to the Ph reading I'm using to?

When you add baking soda it doesn't translate to pH the way you think. You are adding bicarbonate ions, which for your pH pretty much stay as bicarbonate ions rather than taking on a hydrogen ion or giving one up. As a result...and I'd be happy to go through the calculations with anyone out there who is that desperate for entertainment! LOL!...your pH will be sitting very close to 8.3-8.5 or so. Once your pH is sitting at or about 8.3 you can add a little or a lot of baking soda and the pH will not budge...it will continually read 8.3. The real question is whether your alkalinity went up after you added the baking soda. Your pH will change depending on the time of day (due to algae most often the cause) as well as other inputs (the rain you received). Hopefully I'll find the time to give a more complete answer in the near future but does this help clarify what you are seeing?
---David

Thanks David. I guess I should have listened in Chemistry class 30 years ago. LOL. Believe it or not they now offer a horticultural class in my daughter's high school (no such thing back in my high school days). I've already embarrassed my daughters by asking if they offer any evening courses for adults... unfortunately..they don't.

I guess for now I'm okay as the values are the same as in previous years. Having said that, my pond plants grew very poorly in my new pond last year. First pond was only 600 gal, but the water was crystal clear and plant growth was good and looked lush near the end. Second pond (rebuilt last year to 1500 gal with skimmer system) had almost no plant growth, despite me buying more plants (mistake contributing to this was repotting all the plants to gravel only). Now I've rebuilt the pond a third time this spring (modifying the plant shelves, new liner, still 1500 gal) and the lilies are doing a little bit better.. but plant growth is still very slow. (It has been a cool spring).

The PH tends to be always high at 9.0 (8.0 at its lowest). Our tap water is 7.5. All the rocks in and around the pond are granite with the exception of 1 small limestone rock which is the waterfall spillway. This year I've also attached a small bog filled with 50% peat moss & 50% sand. Same liner from pond is continued over to bog with a small drop at two points so that water can be easily wicked into the area. (A pile of gravel and rocks separte the dirt and pond water in the 2 low spots so that dirt doesn't make its way into the pond). Water does not circulate in any way through it and doesn't seem to affect the PH at all (I thought it might lower the PH a bit).

Anyways, I'm wondering for the long run, if my PH is okay and if its just what its meant to be for my pond. Or am I missing something or doing something a bit wrong, that could make the pond water quality better. I know not to mess with PH chemical modifiers. But if the fish are possibly making too much ammonia, or if its just part of new pond syndrome, or it there is something safe and natural that I can do to improve on it, then I would like to know. The fish are thriving, the water at the moment is yellow/brown/greenish murky (presuming its algae bloom) and there is a bit of airy floating algae patches on the surface, but no green algae growing on the liner yet. I know I need to be patient, but its been filled since end of April.. and I'm just seems like I'm waiting forever...
Judy

Aaaaah, nothing like the anxiety of hurrying up and wait for a new pond to break in!!! LOL! That blasted biology just never does what it's told! As to your pH, it's on the high side but okay. Many folks routinely operate with a pH that high. There may be some plants and fish that won't work well with that pH but the traditional plants - hardy lilies, water hyacinths, flag iris, water lettuce, hornwort, etc. and the typical fish - comets, koi - should be fine. One of the factors that will impact on everything is when do you measure your pH? If you are doing it in the afternoon then I am not surprised that it is that high - if you have algae then it will have consumed a great deal of the dissolved carbon dioxide thereby driving up the pH. You should see a lower pH if you get up fairly early in the morning before a lot of light hits the water and the pH may easily be 7.8 to 8.2 or so. This is a diurnal cycle that plays out which indicates you have significant algae in the pond - whether the "string" type or "suspended" algae. You can add some baking soda to push your alkalinity (KH) a bit higher but it won't necessarily help that much. I like to keep the alkalinity around 80-110 mg/L (typically about 6 drops with my kit). The big thing is getting something in there to suppress the algae. Some use barley straw (mixed results); some use UV lights for the suspended algae. I prefer to use water hyacinths to shade and out-compete the algae for nutrients; plus I throw in a few Daphnia (sometimes called "water fleas" - they're not a flea at all!) to chomp on the algae. On the spreadsheet you should find a section that allows you to calculate the amount of "stump remover" to add to your pond. This is nothing more than potassium nitrate which will help stimulate your plants without adding phosphorous. I add a rounded tablespoon about once a week -- my pond is somewhat larger than yours. The dose will not hurt your fish (it's not significantly greater than what is created when they have a heavy feed).
---David