We often wonder how some tiny "harmless" fish can be a problem or impact natives negatively (or at least I do) but recently I caught a few Bluefin Killies in a local lake, they are not native and seemed harmless enough. I put them in my least killifish tank where i had a breeding population of hundreds, a few weeks later and the bluefins have devastated the least killifish population. only a handful are left.
Even tiny exotics can cause problems..
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