I saw an answer to a question about CO2 addition, and it states that fish themselves can make water from rain acidic (I assume they do it by breathing out CO2).
Now I am thinking about my small pond outside (in an old style metal bath tub) which gets its water from rain only. (Placed beneath the rain reservoir I fill always rainwater into it, if even needed in very sunny periods). I assume there are around 200 liters in the pond, and some water plants over and under the surface. Because I was worried about the O2, I added some normal bubbling air pump, especially when the plants were not full grown.
Is this setup risky for my two goldfish?