Aquariums and Fish Keeping

I live in EU where fish medications can be hard to get by. Therefore i treat most issues with salt. The question is:

Can i use salt for water treatment systems in fish tanks?

From what i see, it is pure salt although a bit larger crystals than regular salt.

I have been using salt for ponds up til now, but it can be hard to find, especially during winter seasons, and also fairly expensive compared to filter salt.