Theres recommendations for minimum shoal numbers for a reason. If the minimum to feel safe and content isnt met, is it really keeping them properly?
I could put a single cory fish in a 5 gallon. Sure, the size of the single fish would work fine for the bioload. But now the cory isnt kept in enough numbers, so doesnt feel secure and will get easily stressed. The fish may eat, swim, "play" every day and look totally normal, but stress can be internalized and effect the fish's immune system and should an illness come up, stress will make it so much worse.
So the recommendation from experience is to meet the minimum requirements for their social needs too. If its recommended to keep gold barbs 8-10 minimum, and are then only keeping them in 6... isnt this like that lonely little cory in my example?
Given a full shoal to meet their social needs will make them happier and healthier, while they will also need more space, hence the tank requirements.
(Also, spruce pets is not a helpful source. Ill trust it better once it stops telling me a male dwarf gourami is female
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