Are these fish breathing heavily, looking up while at the water surface? If so, that is a classic nitrite poisoning sign, in which case a ~95% water change is in order ASAP.
Has a bonded pair or even several pairs formed in the tank, which are now giving the "batchelors" grief?
How big are these fish now? I would expect even baby Angelfish to need a larger tank than a 60x60x30cm tank after ~7 months. This could be a case of simple claustrophic stress fighting.
It wasn't going to the surface as much as it was just being in head-up position. The gill movement was a little more rapid than normal, but not exactly what you'd call "rapid."
Yeah, these fish have grown larger since I got them, and I did recently have to put through a 2/3 full tank due to the water going out.
They haven't bonded yet.
My 3 gold angelfish are ~nickel size. 3 of my blue angelfish are ~quarter size. My three black ghosts are ~a half dollar size. My sick blue angelfish is also about a half dollar size. My last 2-3 angelfish are either all three dollar size, or [2] dollar size with [1] half-dollar size. Just checked that lol. Yes, they're about 7 months now.
The over crowding is stunting them also. You said you had it set up since summer and you still have nickel size angels? That is wrong. I have fry that were from eggs laid in November nearly that size...Never mind buying them in a store and bringing home. That means they are probably close to a year old and nickel size is completely wrong for that age.
Are you filtering the tank? How often do you do water changes? Have you tested your water? What are your readings for Ammonia, Nitrite and Nitrate?
I would move these ASAP to a large tank, 5+ feet if you count on keeping all of them and at least 4+ feet if you are getting rid of some.
Hatched between mid-June and mid-July, and I was a little off on the size. The 3 golds are still quite small, but I have a feeling they weren't cared for properly when they were smaller, as there wasn't a sizeable batch at the LFS, and they haven't been growing much since then (though I also think they're a little younger than my blues and black-ghosts.
Yes, filtering the tank (I'm not sure how many gph, but it's a standard 20-40/30-50 filter.) Water changes ~weekly, though I was out of state for 3 weeks, then the water went out almost immediately after I got back, so they were neglected in terms of water care.
Water results:
PH: ~7.4
Ammonia: ~0.25 ppm (since I just added ~10 gallons of fresh water after the water being down, I'm assuming this is the culprit.)
Nitrite: ~0 ppm
Nitrate: Either 10ppm or around 25-30 ppm.
Something tells me that this occurrence is a triple-whammy of an ammonia spike, room constraints due to the water having to be lowered for almost a week, and room constraints due to some of the fish just plain getting bigger.
ATM, the grave angelfish is moving/ "levitating" around more-or-less listlessly around the tank. The discoloration (almost like bleaching) is definitely from an ammonia spike, and they've definitely been getting nipped at. They look a little thinner than their tankmates, and movement is almost entirely dedicated to trying to get away from nipping room-mates.
I think it'd be about time to split them into two groups, putting approximately half of them in a 29 gallon I can spare.
I'm not sure how to help out the sick angelfish. I've been mostly leaving the lights off to calm down the tank mates, to slow down the harassment.