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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?

old style metal bath tub with goldfish in a fish tower

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Looks good. Apparently, you have some substrate for the plants; this has provided some buffering. However, I would check the pH to be sure. I accidentally killed fish once when I put many fish in a clean plastic bucket of rainwater, with no buffering. It became too acidic in a few hours from fish respiration. A possible problem is heat from the Sun, the aeration off sets this partially but I recommend no direct sunlight. I have a few outside containers that get a little sunlight, but they are 24" (60 cm) deep and covered with duckweed; they reach near 90 °F (31 °C). My small Koi pond with less sunlight gets to 85 °F (29 °C). Heat could be a problem. You are also relying on buffering to control pH to avoid corroding the zinc off of your galvanized tub.

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  • Thank you for the explanations. The plants are in pots with soil, if this makes any difference to a "whole floor covered by soil" setup. Jul 5 at 9:38
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    In my indoor aquarium, I use "coral gravel" as a buffer. Keeps the pH in our soft-water area nice and high. I don't see any reason why something like that wouldn't work here, too.
    – Pam
    Jul 15 at 10:32

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