30+ 3-4" fish in a 55? Seriously? Your talking 90-120" of fish in a small tank, bio load would be unimaginable, and everyday huge wc would be needed.....I'd say Strollo had a better ballpark, better to understock then over stock, maybe 5-15 exos.U can do a lot more than 5 exo's in a 55g. More like 30+ most don't exceed 4" in captivity.
A lot of Serra species of piranha for life.
And no serra deserves a 4'x12" tank, piranhas are active species and need space, you said for life, maybe a growout is fine. Now if the op had a 75 that would change the ball game drastically!
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