pH swings are very hard on fish and it is the buffering that keeps pH from swinging around. the test you need is a kH test, or total alkalinity. Alkalinity is the factor that buffers your pH. Ideally, kH should be over 60 to buffer properly. Have you tested your kH?

Limestone can be a good thing, but after a while algae grows on the blocks and pretty well seals them up from further leaching. My tap water has a kH of zero and a pH of 5.5 to 6.0. I put a bunch of oyster shells in a shower filter to raise the pH and to buffer and it;s working great. But if your pH is already high enough,the oyster shells won't disolve into the water. Some people use regular additions of Baking Soda to buffer pH and that works well too.

I agree, oyster shell works great for me. I went to a local feed store and got a 50 pound bag of crushed oyster shell that they sell for chicken grit. Also, at a better feedstore, I saw bags of pure powdered calcium carbonate. I was excited about that, but I already have the oyster shell in my waterfall lagoon.

When I first put the crushed oyster shell in, much of it dissolved. Now, it just sits there because my water has all of it that it needs. Every once in a while I reach in and stir it up to make sure plenty of surface is exposed to the water.

Oh, and the 50 lb. bag cost about $3. That's a tiny price to pay for PH peace of mind. No expensive chemicals, and its toss it in and forget it. Doesn't get any better.

I have a 18,000 litre pond fed from borehole water and the ph is 6.4 and many fish are losing colour so I like the idea of oyster shells as it sounds a very natural and economical way of raising the ph.I was thinking of making a stainless steel wire basket to contain the oyster shells and placing this on a ledge at the bottom of a waterfall so the water would cascade onto the oyster shells.
What do you think of this idea?
How much do you think this will raise the ph ?
Any other comments/suggestions will be welcome

I've been keeping fish and pondkeeping for 40 years,and the easiest and most effective buffer I've found is plaster of paris. I make big tablets in 5 gollon buckets about 3" thick. I pop them out of the bucket mold and I toss one into the filter vat or in the bottom of the pond near the waterfall or an airstone, so plenty of water circulates over it. When the tablet is about disolved I just toss in another one.It works better than oyster shell, crushed coral, dolomite,etc...
Try it you'll like it.

My last post about the plaster of paris is not some kind of hooey that I invented. When I was a kid way back in the 1960's all of the pet shops and variety stores, that sold fish keeping supplies, sold a product made of plaster of paris to keep in your fishbowl. They were called various names, like Water Conditioner, Molly Blocks,etc.. Some were called Weekend Feeders, with little pieces of fishfood in them. They were mostly molded like little seashells to be decorative. The main thing they did was buffer the ph of the water/ raise the alkalinity of the water.
I never understood what they really did for the water until years later when I started keeping salt water and african rift lake fish. They all require high and stable pH water, and it was maintained by using alkaline gravel in the tanks (kind of like adding lime to your garden soil). This only seemed to work if you did a lot of water changes, which are still very important, but I remembered those little blocks of POP and made one for my aquarium. Suddenly the pH was very stable, and I have used them ever since.These work so well because the POP is less stable in water(disolves faster than the gravel).
My freinds that have pet shops use them in the filter vats on their systems to.
Garbird

I agree that the POP is the best thing to keep the water PH steady. It only dissolves as much as is needed. It is a chemical reaction to the acidity of the water. It is very noticeable after a heavy rain. I got tired of running out to dump a cupful of Bicarb in the Skippy every time it rained. If you plan to use the slugs or get fancy with various shapes remember to use only POP powder that has no additives. DAP is one readily available plaster powder that is free of other chemicals. I use a couple of boxes a year. (I like to make bubbles out of POP in different sizes.) Sandy

While I have never needed to use it, garbird's plaster of paris suggestion is indeed a time honored remedy.