If the approved use scope of your registered trademark is only mineral water, but there is no pure water, then strictly speaking, you can only use it as an unregistered trademark on pure water, and you can't use the registered trademark mark.

Just because you registered a trademark on mineral water, and mineral water and purified water are similar commodities, even if you don't register, the victim can't register the same or similar trademark on purified water. So you use it as an unregistered trademark and don't have to worry about trademark infringement disputes.

Add two points to my friend's answer upstairs:

1, there is no problem to use, but it must be clear that it cannot be used as a registered trademark, otherwise it is strictly a registered trademark.

2, it can be used not because it is a big class. What matters is the similarity of goods/services. Not all goods or services in a large category are similar goods/services, and goods or services not in the same category do not necessarily constitute similar goods/services. In this case, because mineral water and purified water belong to similar commodities, others cannot register the same or similar trademarks on pure water, so it is no problem to use them as unregistered trademarks.