It has happened in my hard water....One of my tanks was suffering like that for ages...all snails turned white, hard water straight from the tap as in my other tanks....
Water in fish tanks is subjected to acidification by nitrification and other oxidising processes...The Ca++ gets used up and although a GH test will still show your usual reading the ions have been oxidised and are not useful to the inhabitants anymore. Gh tests can't distinguish between the two....Hence in an old not water changed tank one always gets a high Gh reading due to evaporation and top ups without water changes but it does not mean any of it is of use to the system and in reality the water is quite acidic, normally low Kh and even low Ph in time...What's your substrate like, perhaps it is not inert or is full of organic matter, or there's too much bioload acidifying the water... I'd try to do a good reset by several water changes and a good cleaning because that's what normally is the issue...
