If you know someone with baby fantail goldfish, you could put 2 tiny ones
in a 5 gallon. Plant the tank with live plants, Then feed only 1 flake of
food each/day, with a partial water change only twice/month.

I would suggest twice weekly rather than monthly to maintain the water
quality......

The reason for few water changes is so the build up of hormones slows down
the growth cycle. ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us