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In general, fish can adapt to any pH(within reason...they can't live in a pH of 3.0, obviously, nor could they live in a pH of 10.0) or water hardness. They are, however, adversely affected by pH, hardness and temperature swings. All of these things cause changes in their biology that can be very stressful and possibly fatal to the fish.
pH and hardness go hand in hand, and when they change, it causes in osmotic shift in the fish. That is, the fluid inside the fish tries to equalize with the water outside the body. As I stated, this can be very stressful or possibly fatal to a fish. Lets equate it to you soaking your hand in rubbing alcohol for 30 minutes...take your hand out, and your skin will most likely be burnt, or at least extremely dry and uncomfortable.
So, with that stated, SW cichlids can live in hard, alkaline water, and rift lake cichlids could live in soft, acidic, tannin stained waters. You just have to acclimate them slowly, and be consistent with your water parameters. This is the reason adding buffers/chemical softeners to water is not advised by most knowledgeable fish keepers. It causes your pH to become unruly, and hard to keep a handle on.
This is how I understand it. If I am wrong in the least, someone please correct me.
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