Many new fish friends always struggle how to change the water and change the water problem on the number of. Change the water hard or not is relative. Some day changed, some for a week, some every two weeks-for even longer. This is calledall roads lead to Rome, all are reasonable.
Often, and enhancing adaptive capacity for the fish, but the fish will be very tired.
Not often, and fish are delicate, but safer. Key is to observe, fish are uncomfortable to fish, because fish where there are problems is to rely on the usual accumulation of. It's hard to explain. But the water is suitable for, are priority issues to be considered.
In other words, change frequency should be flexible, so dogma has backfired. Buyback, sometimes mysteriously died, I believe most of the fish are gone, was brought back from fish farm bacteria is possible, so don't blame yourself hard water.