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At 8 months, they're likely hitting sexual maturity. The aggression could be breeding behavior. If you don't have a pair, the dominant one might be asserting itself. Monitor for pairing off or egg-laying.

Aggression in angelfish can stem from territorial behavior, especially if they're mature. How old is the aggressive one? Also, what's the male to female ratio? Sometimes adding more plants or rearranging decor can break up established territories.

One of the good things about Azolla filiculoides is that it doesn't have those pesky invasive roots. It's actually pretty easy to keep contained, way more so than duckweed. Plus, it's a great source of denitrification for your tank. If you're looking for something similar, Ricciocarpus natans is ...