I guess you could put anything you like - it's your tank after all. But I personally would not. I had ~7 Neons in a 12g long and apparently it still was not large enough for them - they basically just stayed in one place facing the current and rarely moved from that spot - pretty boring. I re-homed them into a 40B and now they are all over the place.
A 12 gallon long is 35 inches while a 40 Breeder is 36"
Why wouldn't they shoal in a 12 gallon long?
I did some recalculations
It seems my dimenions were off
5.5 gallon = 160 sq inches 16 Long
10 gallon = 200 sq inches 20 long
fluval 161.7 sq inches 16.5 long 16.5 x 9.8 x 8.3 = approx 5.7 gallons (so it is closer to a 5.5 not a ten, though it is wider)
I think AqAdvisor = inside dimensions which are more accurate.
My bedroom is small. Not much space. So a fluval is perfect.
I know they won't be able to shoal, but they can group together.
Would they die in a fluval if all other things are in order? I have seen reports where people with planted fluval can overstock once the cycle is running.
I know they cannot shoal like schools for a length.
Cardinals are a thing of mine. They are the only tropicals that come close to the color of saltwater. At least to my mind.
I do not mind if the cardinal tetras sort of congregate. What I do not want to be is cruel.
I have seen them in smaller tanks and they seem okay.
There is not way, at this point in my life, that I will ever get a 40B.
Anyhow, how would my plants be with stock lighting.
Small driftwood Not boiled NOT soaked with java moss glued to it. Maybe a small fern glued to it
1) Anubia (dim corner) (can you recommend a slow growing one)
2) Java Fern (dim corner) These are slow growing
3) Blyxa Japonica (under light) Fast, but it is where I can trim it easier
4) rotala indica front corner I do not know how fast this is.
5) Marimo Moss ball in front corner Maybe 2
I want this low tech.
six cardinal tetras, 2-3 CPDs, and an oto, are technically overstocking, but these are low bioloads.
That is 9-10 fish, but may not produce the bioload of 2 platies or guppies, or even one pea puffer which I am told is a micro fish but a macro mess.
The fluval is so elegant but so poorly designed. I have to write to hagen with some ideas.
They could have made that hole and light bigger. The could have angled the hole rim so that the light would fan out instead of being contained.
They could have protected the lighting better so an air stone could be used under the hole without frying the light.
The fluval edge is beauty in a tank, but it violates every rule of good tank design.
1) Not enough oxygen area.
2) poor lighting distribution.
The only thing it has going for it is a good filter, an oversized AQ20.
Even then you have to put a rock under the filter to prevent it from washing away soil or sand underneath it.
One guy on Yahoo (I think) said Cardinal Tetras stress out and die in small tanks in a year. I doubt that. I seriously doubt smaller length kills them. I suspect poorer water quality.
But the AQ20 would take care of a lot of that. The plants would take care of more. And I would consider probably a half gallon water change twice a week or more.
I think AQ advisor is too conservative. Their figures are based on no plants. Plants are natural filters.
Anyhow ... how are the plants I suggested to an edge?
Do cardinals die if they cannot shoal? but all other things are in order.
I may have to go to to a ten gallon which I do not want to do. I have a table which is perfect for an edge ... or a 5.5 gallon; but is only 18" wide and too short for a 10 gallon.
I still do not understand why a 12 long was not suitable for you and your cardinals.
I know someone will recommend scarlet badis, but they are finicky eaters. I do not want to spend $10 a week feeding them specialty food when cardinals will eat flakes.