There are two commonly used pH test kits: 1) paper strips ; 2) liquid drops in a vial. If your pH test kit are the paper strips, then be aware it is only accurate within a +/- 0.3 variance, which makes them quite inaccurate. Paper strip test kits are more used to serve as a "fire alarm", that is it only signals there might be a fire. The "fire alarm" can definitely be wrong or it can be correct except it does not give a good accurate indicator on the fire. Paper strip test kits are quite fast, in and out, 5 second or so, quick indicators to tell you if the "alarm" is triggered. If there is a possibility of concern due to the paper strips "alarming" reading, then the liquid drops in a vial pH test kit is used. The liquid pH test kit has an accuracy of around +/- 0.1~0.2, which makes it a much better test and should only take about a minute or so to conduct.
If you are using a liquid pH test kit, then this would mean your water actually is, while including the variance...
Morning = 8.1~8.3 (or 8.0~8.4)
Evening = 8.7~8.9 (or 8.6~9.0).
Your pH fluctuation might not even be due to alkalinity.
** Alkalinity (above 7 dKH or above 125ppm) helps to stop your water's pH from falling below 8.3~8.4.
** Free Calcium helps to stop your water's pH from rising above 8.3~8.4. The volume of free calcium required depends on many variables such as plants, contanimants, etc, so this is not easy to pin point.
** Quick asterisk. There are many different compounds in water that determine its pH buffering abilities. This context of alkalinity and calcium assumes your water is not heavy in phosphorous due to underground aquifers with heavy phosphate rock formations or the water comes from springs or streams running down a mountain that is heavy in phosphate. The one area I know of that this might be an issue for is for some people who live in North Carolina.
However, most folk's ponds are not heavy in phosphorous. Most folk's pond water is more heavy in carbonates, such as what is created due to living aquatic organisms expelling carbon dioxide including algae and fish, etc.
With a morning pH of 8.1~8.3, then this tells me your alkalinity might need a little boost, except not much at all. This is why sharing the KH test kit values, as described in
post#13 of a pH & KH thread, is quite informative.
With an evening pH of 8.7~8.9, the this tells me you have little free calcium available in your water, which is solve by using calcium chloride as described in the
Clacium Chloride to lower PH thread.
Without knowing your KH values and assuming your pH test is correct, then...
Personally, due to your pH fluctuation, I think your water's alkalinity is not low enough to be too concerned about just yet, but you still need to know the KH value, keep an eye on it, and maybe toss in a significant amount of a calcium carbonate product, such as oyster shells, in an area of the pond where there is moving water, a stream floor, a bog, base of a waterfall or fountain, etc.
There are many good ideas of how to integrate the above mentioned product in the threads of this chemistry sub-forum. Remember though, oyster shells stop dissolving at around 8.3pH; although, if the pH ever dips lower, then a small amount absolutely will be released into the water. However, if money is a problem, then just skip it; calcium carbonate or aragonite or crushed coral or other formations of this product is just a good preventative product to use to help ensure your water is healthy.
Although, free calcium levels might be too low.
Your source water likely does not have enough free calcium to satisfy your pond's demands. Simply follow the instructions about calcium chloride, as mentioned above, and then wait a day or so, test again... If the 8.7~8.9 value has not droped down to around 8.4, then repeat this process 2 more times and spaced out between each dosage... If after the 3rd dosage and still no change, then do not dose anymore and be sure your liquid pH test kit has not gone "bad" due to passing the expiration date. If you are positive your pH test kit is correct, then I think the calcium is not low enough to be concerned about just yet, but keep an eye on it.
These are just the recommendations that I personally would do if it were my pond.
Absolutely nothing about these products will harm your fish nor plants as long as they're used in moderation as mentioned above.
To reiterate... I think your pH is border line on being quite fine, even with the fluctuation. Just keep an eye on it and pay attention to your ammonia test while making sure there is no ammonia or your ammonia test stays below 0.30ppm.
