there isn't really a way to calculate how many per gallon all fish have different habits and get to different sizes, some are swimmers while otehrs are more dormante, some graze on algae others need meaty foods.
if nitrates are low and things are not full sized and to cluttered you could look into adding more, but pay attention to their eatign habits and how much your skimmer is pulling out as well as your nitrate levels
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