11-01-2012, 06:28 PM
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As it mentions in our profile of Hemigrammus bleheri (the brilliant rummy), this fish always does better with larger groups. In your 3-foot tank I would aim for 15 of this species.
Cardinals are a bit different, and a group of anything above 5 should be fine. In their habitat, observations have noted that in those with more open space the fish remain in huge groups of hundreds and stay close. But in habitats with sunken branches, aquatic or marginal overhanging vegetation that is thick, the fish tended to break up into smaller groups of 5-6, though still remaining close to the rest. And as Sherry noted, the tank size does impact; the more space, the more they congregate, which is obviously connected to what I just mentioned about their habitats.