Answer: I am unable to find any reference during a quick search to the specific effect of four o'clock seeds (mirabilis) on fish. However, it is possible that any accumulation of organic material could contribute to a decline in water quality over the winter.
It is also possible that your fish have grown and multiplied and are now more crowded than they used to be, thus increasing the stress and potential gas build up during the winter. You might want to check with your favorite fish specialist and see how many inches of fish and in what combination your pond can realistically be expected to support.
I'm sorry I can't be more specific.
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