[Pavel Chaloupka]

Yeah Bernd is right. The main factor you have to take in to account is that the water you will be using for the exchanges has no ions that could make the pH higher, you only dilute the environment which happens on the natural localities for xample during heavy rains. Even in the nature the pH value will vary slightly during the day so water exchanges does not make nearly as big of difference as they would if you would use tap water in normal tanks. And you can prepare the water for exchanges, which make things even easier but as Bernd said, it is not necessary. As long as you add water that contains no alkaline substances, the pH will stay within the boundaries.