Dendrobium leaf issue?

[Sienna]

Hi,
I am new to the Orchid Board, and this is also the first time i have used a forum

I recently recieved a bulk lot of Australian Dendrobium orchids and i am a bit worried about some makrks on the leaves. They are all on different plants.

Does anyone know what causes them and what I can do to treat it?

I have some Previcure fungiside i can use, but I tend to avoid this where I can.

You can see in the photos that the tap water is causing a build up on the leaves, we are in the process of installing a rain water tank so i can give them fresh water. Any tips for dealing with this in the mean time would be great.

Thanks