That general rule applies for only certain fish or certain sizes of fish as we know that common goldies don't go into 20-30 gallon tanks ;) but that is a good idea to keep in mind for guppies, but i'd expand it to gallon-per-inch-plus-one-more for guppies because they are little poop machines themselves.
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Fluval Chi: Freshwater setup, home to a double tail plakat and a hunted colony of copepods. the guppy thread! 30+ pages strong!
Another one of my guppies is having babies, she just had 6. The other one stopped at 20 or she ate some when i was sleeping because there was 20 when i went to sleep and 20 when i woke up. 26 frys now lol. What am i gunna do....