I agree. People actually add limestone to ponds to raise the ph. Koi and goldfish prefer high ph between 7.0 and 8.6. The maximum level that limestone can alter water is 8.3 I can think of only two reasons that limestone might create a problem.
1. It was already contaminated with something else.
2. The pond is overstocked which causes high ammonia levels. Ammonia is extremely toxic to fish and toxicity increase as ph goes up.
