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I have on angelfish in my 55 gallon tank. He's a standard color morph (silver with black stripes) and I have had him for probably 4 years now.
Is there any issue with introducing some smaller angelfish in the tank with him?
The tank also has 7 neon tetras, 2 flying foxes, and a male and female betta.
I'd like to get 2 small angels to add to the tank, but I wasn't sure if this guy might get aggressive towards them.
I probably should also sex this guy.
if you decide to go this course, add them in groups of 6 juveniles. Larger dominant cichlids rarely see smaller juvenile fish as a threat. If you notice one getting picked on, place him in a breeder net (preferably without changing tanks to add more stress) or other quarantine area as the targeted fish will likely be either a rival male reaching sexual maturity or a female in the runners-up position if your tank boss is a girl.
Continue daily observation as frequently as possible more than once a day if you can at this point and focus on aggression and the fish that receives it. If all goes well you can see the dominant fish to a harem, if not you can either add more fish again if you have more means of quarantine or continue removing fish that get attacked and rehome them. The goal is to ultimately have 1 dominant male and as many females as he will tolerate and your filtration/water change regiment will support.
A group of the larger growing Corydoras such as Emerald or Peppered or Paleatus would be appropriate to redirect aggression from Angel to Angel since Cichlids will not target a fish that is not another cichlid in most instances. Try adding the Corys at the same time for maximum success rate and acceptance by the dominant fish. Contrary to popular belief, those species mentioned do just fine with gravel - a sand substrate is NOT needed for the hardier, larger corydoras species.
If the Bettas become an issue, remove them and keep them separate while focusing on turning the tank into an Angelfish habitat in every way you can. The Bettas can be isolated quickly and split up therefore all of your attention should be focused on the 55g once this is accomplished. Other than that, you should be ok as long as you keep constant watch.
Various Dithers can be used while introducing the new juvenile Angels besides the Corys such as Tetras, Danios, Rainbows, Barbs, Rasboras, Rainbow or Redtail Sharks (and with discretion due to the Bettas - groups or harems of Molly, Guppy, and Platy).
I could also see Pimelodus pictus working in your favor as well if you decide to try some more bottom-feeders, as it seems your tank lacks them FTMP. A solo Pictus will appreciate the company of other active catfish like Corydoras and school/shoal together as if they were the same while keeping your tank extra-clean.