Dawg you are hilarious!!!
Depends on what Mbuna you are speaking about, if you plan to have a breeding colony, if you plan to have all males, what else you plan to keep with them? In general about 20 would be fully stocked but be more specific cause "mbuna" is pretty general.
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"There he goes - one of God's own prototypes -
a high powered mutant of some kind never even considered for mass production.
Too weird to live, and too rare to die".
Fear and Loathing in Las Vegas