so my questions are:
Is the aggression normal?
What would help?
How to make sure the new fish are eating since the had and tails are more brave, and are pigs?
How to keep track of the number of neons? I want to make sure their school stays above ten, but with my spotted Raphael, if they die I probably wont find them.
Side note. The tetras are full of color while hiding in the plants, but tend to fade when out in the open. Is that normal?
I haven't kept head and tail light tetras before so I don't know for sure if they are normally peaceful or not. But a quick search and it seems they are peaceful (with possible fin nipping to long finned fish), so no, the aggression toward the neons is not normal.
I didn't see anything mentioned about them being territorial, so they still wouldn't harass other fish even if they have been there a long time and just now see new fish introduced. I have introduced tons of new fish to tanks with other fish that have been there for a lot longer and no harassing just because they are new (again, they aren't territorial fish though).
Unusual aggression is due to stress factors, causing them to be irritable or just unnormally mean. There are many possible stress factors, such as improper water parameters/environment, issues with tank mates, disease, even excess lighting, or lack of hiding areas, etc... But right away, what stands out is the small group of head and tail light tetras. Only 3 is too small of a group and causes stress among those fish which can abnormal aggression. At least
6 is recommended for any schooling/shoaling social fish. So what would help is getting more h&tl tetras. I would recommend for you to get at least 8 in total.
Your concern about the h&tl tetras harassing the neons to death is very valid. The neons being new and already stressed, they indeed can die from the additional stress the other tetras are putting on them. Depends on the severity of the harassment though.
Lot of ways to increase the likeliness of survival, but that's a whole list of things and I'm too tired to type it all out right now. Pretty much just do anything you can think of to minimize stress if you want to the best chances at success. Leaving the light off is one thing.
It's not uncommon for new fish to not eat for a day or two when first introduced until they calm down/settle into their new surroundings, currently they are just too stressed. But if they don't eat for much longer than that initial period, I would worry. As mentioned, stressed fish tend to not eat, but I would address those stressors, whether that is the other tetras harassing them or not.
Seachem GarlicGuard can be added to fish food to make it more attractive to fish, plus it has health benefits as well. Lure the h&tl tetras away from the neons with food while also spot feeding the neons so they have more of a chance to eat.
You did good with an appropriate sized neon group of 12 though, as they feel less stressed/more safe with a larger size group.
Pretty much the only way to keep track of the neons is to count them (they will probably be in a close group, so it should be easy, especially seeing their iridescent blue stripe).
The fading when out in the open isn't really normal. Once they've been exposed to the light for a little while, their colors are "developed"/present. Maybe it's just a optical illusion/effect you are seeing when they are against different "backdrops" (green/red plants vs out in the open)? Or it's just the way/angle the light is hitting them?
Regarding the other LFS, I would call first just to make sure they have them as that is a long drive. Fish can survive a 6 hour drive (they fly for longer, even without pure o2). Would help if you told the LFS how long of a drive so they can make sure they put more oxygen in the bag -then again, water volume affects pollution dilution potential :P. I would put the bagged fish in a insulated cooler/styrofoam box to keep water temp more stable -especially from heating up which would further reduce o2 in the water- and them being in the dark reduces stress. Lot of other things you can do to minimize stress, just think about it.
Man, I always type so much. Well...I'm off to get some rest :P