Most Aggressive Reef Fish (Add Last)

An empty tank reads as unclaimed territory to the first aggressive fish you add, per Top Shelf Aquatics. Add these species after everything else is established, and expect the mitigation to reduce aggression, not eliminate it entirely.

Damselfish (Domino, Blue Devil)

Domino damselfish and Blue Devils sit in the "highly aggressive" tier, recommended to wait 14-20+ weeks of tank maturity versus 8-10 weeks for peaceful Chromis, per the same Top Shelf Aquatics guide. Even with correct sequencing, a Reef2Reef discussion lands on "helps but doesn't fully solve it" as the honest consensus.

Six Line Wrasse

"Add the six line wrasse last. If you add it first, nothing peaceful survives after," per Aquarium Store Depot. It can harm small gobies, firefish, and other wrasse species added alongside or after it, and its behavior tends to worsen over time rather than settle.

Dottybacks

Dottybacks "will bully not only each other but their tank mates," and larger fish aren't safe from being chased and nipped, per BuildYourAquarium.com. A bicolor dottyback will attack a royal gramma "mercilessly" purely for looking similar, regardless of the gramma's own temperament.

Triggerfish

Picasso, Undulate, and Queen triggerfish are each individually flagged as "very aggressive" to "incredibly aggressive" toward fish and invertebrates alike, per Aquarium Store Depot. Most triggerfish are better suited to species-only or predator-focused setups than a mixed reef.

Tangs and Foxfaces

Tangs need swimming room and can bully newcomers, generally recommended only after 12+ weeks of tank maturity, per ReefBay. See this site's own Tang Compatibility guide for species-by-species detail. Foxfaces are generally peaceful, except toward other rabbitfish and tangs specifically, since both compete for the same algae-grazing niche.

Maroon and Clarkii Clownfish

Maroon clownfish are "the most territorial clownfish species" and will attack fish much larger than themselves defending a host anemone, per Top Shelf Aquatics. Clarkii clownfish grow to 5-6 inches and become increasingly aggressive with age, straining coexistence in a limited-space tank.

The bottom line

Add territorial and aggressive species after everything else is established, with 1-2 weeks between additions so each fish's behavior settles before the next arrival. It reduces conflict. It doesn't guarantee peace, so watch closely for the first 24-48 hours after every addition regardless of species reputation.