Fish Friendly stone
If your looking for aggregates or rockery stone for your fish pond then you should be careful with your choice. Many fish are sensitive to changes in Ph levels in the water. Some types of stone can effect the ph of the water.
By a general rule of thumb only metamorphic or igneous rock types should be used like granite or slate.
However we have tested some of our limestone rockery stone and sandstone rockery stone. These are sedimentary stones and technically they can alter the ph of the water. Our tests showed that if the stones were washed first then they could be used and no effect was observed in the ph levels of the water over 24hours. The surface area of the rockery stone is probably so little in comparison with the water that it didn’t have much effect. Using decorative aggregates or pebbles which obviously have a greater surface area by volume did increase the ph in the case of the limestone. Some fish are safe in alkaline water (a ph of over 7).
It is obviously best practice to wash all the stones thoroughly before using in an environment with fish. If you are wanting to use a sedimentary stone then it would also be worth purchasing some acid test strips and leaving the stones to soak over night in a bucket of water and testing the ph after 24hours. Then researching your species of fish you can determine whether or not they will be happy living in the ph indicated on the test strips.