That sounds interesting. I would wonder if they dissolve eventually or if bacteria could tolerate the high PH. It seems to me it could be very expensive compared to the many items that have been suggested. Maybe you have some sort of cheap supply source?

I have found Plaster of Paris works very well to maintain PH levels but it doesn't go into the filter.

Sleepless, plaster of paris is gypsum, which is pH neutral. It will supply calcium and sulfur, but it won't affect pH.

Jim, I think a bag of crushed coral from a fish store would be a better choice. Coral is naturally porous, and the smaller particle size would provide more surface area in immediate contact with the water. Also, cuttle bone contains organics that may or may not be desirable.

Use of POP (Plaster of Paris) is mentioned in this explanation of various chemical issues including buffering. The point of using POP is that it is neutral. It only reacts chemically with acid (below 8.3) and stops reacting once 8.3 (or close enough)is reached.

Other posts found in a search are very helpful. KH and PH are often confused and I should have made my statement more clear.

Here is a link that might be useful: Maintaining buffering capacity

In that link you state that plaster of paris is calcium carbonate. But a quick search shows that plaster of paris is calcium sulfate. If it were calcium carbonate, then the carbonate would buffer the pH as you say. But calcium sulfate has no such effect. I couldn't find any examples of plaster of paris containing calcium carbonate, but maybe it is sold that way somewhere? In any case, there are plenty of other sources of calcium carbonate: dolomitic lime, crushed coral, oyster shell, etc.

There is indeed a calcium carbonate plaster of paris even if most of what you see is calcium sulphate. The following information may be helpful.

"Posted by drh1 z4 VT (My Page) on Sun, May 17, 09 at 18:28
A few comments: if you decide to make the plaster-of-paris pills or tablets be sure that you are buying POP that is made with calcium carbonate. Some versions of POP are made from silicates, won't dissolve worth a hoot and will have no impact on your buffering, pH and little on your calcium concentrations. How to know? A. Check the ingredients if listed. B. Use a small vial of vinegar and place a drop or two on a little bit of the POP...if it fizzes then it has carbonates in it. The DAP stands for the company that makes the POP and is not some weird acronym for it's contents. Sodium bicarbonate is used by many to adjust pH and more importantly, alkalinity, commonly referred to as KH. The strips used for testing, as Sandy has mentioned, tend to be inaccurate and have a bigger problem in that their shelf-life is rather limited compared to the liquid test kits. I've benchmarked both against the gold-standard titrimetric method in a lab and found the liquid ones to be more accurate. The primary advantage of sodium bicarbonate is that it is one of those chemicals that relatively benign in terms of potential problems of putting in too much....once your water pH is at 8.3-8.4 whether you dump in a cup or 10 cups will have no further impact on pH although it will increase the alkalinity. This is NOT true for pH PLUS from swimming pool stores. This product is typically sodium carbonate, not sodium bicarbonate. As you add this stuff and happen to over dose you WILL raise your pH. In fact, it is possible to raise the pH as high as 11 by adding just sodium carbonate...you can use it but be careful. Same goes for using muriatic acid...it can be diluted and used as need be but I think you would be far better off leaving the pH at a stable level rather than trying to chase it around. You will probably find that your pH will vary significantly during the day...low in the morning, higher in the afternoon...just due to the influence of algae in your pond. Bring the algae under better control, plus keep your alkalinity levels above 80 mg/L and you'll have less fluctuation in your pH. You'll probably find that many folks...here in this forum or elsewhere...routinely keep their ponds in the pH range of 8.3 - 8.8 with no major effects on fish, plants, tadpoles, birds, etc. Not true if you're trying to grow fish that enjoy a more acid pH (some African types, if I recall correctly) or plants that are very pH sensitive...I believe there are some native orchid species that may fall into this category, for example.
I used to make the POP tabs but kept running into more frequent problems of locating manufacturers whose products which contained carbonate-versions. Now all I do is throw in a handful or two of sodium bicarbonate (baking soda) once in awhile. Specially add it in the late fall headed into cold weather when things begin to freeze ... want everything well buffered at that point.
Hope all this stuff helps.
---David"

I guess it shows why it is important to use chemical names rather than commercial/industrial names...

Thanks all. I hadn't thought about coral. 5# of cuttlebone runs $20-$25 online. Since cuttlebone is pretty low-density this should take up more space than the same weight of coral gravel and have more of the properties of the scrubby pads that many people use in skippy-type filters. I may experiment with a smaller amount over a bed of coral gravel and see what happens...