09-14-2007, 11:03 PM
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I've heard that loaches should be kept in odd numbers because it prevents them from getting into a situation where the fish form aggression pairs, where one fish always beats on the same other fish.
I could see it working the same way with tetras, though being less necessary. In my 55 gallon tetra tank, I have diamonds, glowlights, flames, black neons, and long finned blacks. Most of the schools are even numbers, some of them are odd, and I don't see any difference in activity levels on that basis.