Not really an issue just something I've noticed as a new fishkeeper I have around 12 cardinal tetras and 6 rummy-nose tetras have noticed during the day they tend to school and swim together
but on a night time after 7 hours of light they all seem to swim individually all over the tank and picking a bit of wood and trying to find bits of wood in the plants is this normal behaviour for tetras?
Not complaining as the tank looks beautiful and full and there's always something interesting to watch I also wondered whether there was a certain light spectrum that would make these fish pop a little bit as my aqua sky 2.0 seems a little bit bright and they seem to get lost in the bright lights.
Re: Runny and cardinal schooling only during day.
Good to read what you've stocked with - and lovely fish. Glad you're happy with the tank and enjoy what goes on in it.
I have noticed that behaviour changes over the course of a fish's/shoal's lifetime and depends on many factors such as layout, decor, other fish (species and individuals) in the tank, and a lengthy list of others - every permutation has occurred at one time or another over the course of their lives.
Rummynoses in particular are known to be shoalers. Cardinals are known to explore the substrate for food. At the moment, in your case, it might be that the fish feel more comfortable and confident to do their own thing after "lights out". The situation may change over time - there's never a dull moment in fishkeeping.
I have some relatively young cardinals (8 months old) and they are still wary of me when I approach the tank, shoaling together. Once they can see me at a distance or if I sit still for a while beside the tank, they do their own thing, including lots of pecking in the substrate and on the wood for food. Their behaviour is changing too - the males have started taking an interest in the females and so pairing off into smaller groups, while the females are more concerned about fighting over food.
This week, the ballast to my lighting died and I opted to get https://www.aquael-aquarium.co.uk/prod ... ting/leddy-tube-retrofit/ 8W Sunny as this was the only option to fit my existing tank lid - you can see the colour spectrum distribution on this page; it definitely doesn't do justice to my (last remaining) harlequin rasbora and has rendered her a copper colour.
The cardinals look much the same under the new light as they did under the previous light.
So, if you can find some form of lighting more akin to the former, that might help you.