Ok so this is going to be a longer sorta post so I apologise
Tldr: mature black neons are species aggressive and no one seems to say that before hand and we'll lesson learned.
So last Friday I finally picked up my 30 black neon tetras super excited and couldn't wait to watch them school for days to come.
For some info incase you haven't seen my multiple post about this tank. It's a 40B planted with Christmas moss, java ferns, brazilian pennywort, crypt wednii, foxtail fern, and a plant I just can't remember the name for the life of me ( very common plant though) two larger pieces of wood. temp is set at 76°, I dose easy green ferts and use co-op root tabs, water perimeters we're all 0 with the exception of nitrates at 10 - 20 ppm, oh was at 7.6, had been fully cycled for about 3 weeks as plants were showing new growth and even pearling.
So went to the lfs that ordered the fish for me, I felt amazing as a large majority of the fish were really good size close to 1.5" maybe close to 2" in size i would say maybe 10 were smaller then that. From what I assume sexing them I had about 8 large females the rest were males. ( I think this was mistake number 1)
Took the fish home left them to drip acclimate for around 30 to 35 mins. Everyone was looking good and healthy so I transferred fish into tank. Instantly fell in love and was swooning over my happy little tank ( insert ominous music here please )
Next morning I woke up and had 3 smaller ones that had passed I didn't think nothing of it cuz 30 fish in a bag for large period of time while shipping then being added to my tank losses were expected. Went about my day as normal came home from shopping with the wife had 2 more smalls dead, again didn't shock me smaller fish being less hardy but I still wanted to check my water as I did just add 30 fish at once. Still everything was fine after testing so I fed the tank and turned lights off for the day. This is where things started getting a bit odd. I noticed the school had now become 2 separate schools one with all the females and majority of the larger males the second the remaining small males and a few of the larger males. The larger school kept swimming in and out of the foxtail darting at any fish from the second school I assumed maybe those large females are full of eggs and they were trying to pair up or show off even though my tank doesn't really have any kind of swan mop but maybe the foxtail they were claiming was there best option so they got territorial over it. Figured they would realize it wasn't going to work or hell maybe lay eggs but that was as far as I though it would go.... Till the next morning...
10 fish were dead and had there fins ripped off laying dead at the bottom of the tank with marks all along there bodies. (Mind you they are the only fish in the tank no others) so I immediately flipped out and thought my water crashed as I was under the belief they were peaceful fish and maybe just picked at them once they died. Tested water which as far as my api master test kit said was perfectly fine yet again I assumed human error and tested 3 more times to be safe... Still nothing so I did a massive 65% water change cuz I was convinced I was doing something wrong while my wife took a water sample to 3 lfs to get tested all three stores agreed water was in good condition for fish. So I was at a complete loss. I assumed maybe a bad batch from the fish farms as it's known to happen or maybe even neon tetra disease then my phone rang my lfs called to let me know fish for my wife's tank had come in. So we went picked those up (I also picked up some fish for my tank which I know most people will say that's a terrible idea as I'm currently loosing fish left and right) came back added 6 albino Corey's and 4 scissor rasbora ( acclimated the same way as mentioned above ) We made dinner ate watched a movie went back to the tank another 4 dead. I am in full on I give up mode I had no clue what else to do.
So last night I decided to stay up and watch the tank, lights off only light was in the back of the room which gave me just enough to see into the tank where I would periodically look to see what the fish were doing. Then it happened to disbelief I saw 4 of the large neons group up and nip the fins right off of another neon. I quickly tried to grab the fish to move to a separate tank to see if It would be ok not even 5 mins later that neon passed. So I go back to watching by 530 am they had taken the pack down to 7 neons 4 males 3 females are all that's left. 7/30 survived. I was truly miffed as I was led to believe they are peaceful fish everything online that I read, people I had talked to everything, then I started searching other forums and pages specifically for black neon aggression in the search bar come across like 4 or 5 others who have had this same problem. Seems once they reach a larger mature size they become territorial and aggressive to there own species....
Ultimately this is a learning experience that I should dive a lot deeper into researching fish just because majority says peaceful doesn't mean they are always going to be that way...
Side note this entire process I was speaking with @xXInkedPhoenixX who also purchased black neons around the same time and has seen some aggression as well. Luckily no where near my level but still aggression all the same.
Now I'm sure there will be a large number who will try to call BS and I don't blame you I wouldn't believe this if it didn't happen to me personally but thought it was worth sharing with others.