OK, so what really is the blue stuff they put in the fish water when they ship some fish???

I got a Dozen Jumbo Cardinals in today, along with a a couple different plecos... Cardinals were in one bag, with the blue stuff, and each pair of plecos, in one bag, with no blue stuff...
I have gotten fish previously with the "blue stuff" in the water... I don't remember specifically, which fish, or sizes...

but the fish looked poorly, about half on their sides... but within a few minutes they were swimming normally in the tank... wondering if it slows them down, either for bigger or hyper fish... being Cardinals, they were not "big" fish, but being Jumbo's they may have been wild caught, & or hyper???

Methylene blue.

It's actually methelyne blue but also a sedative. I've used it for shipping swordtails, or I did back in the day.

Any fish sold at the jumbo stage is more at risk in shipping, as it's older. Young fish ship much better, so I would guess they were concerned about sending the oldies. It's good practice to sedate in such cases.

I'm kind of bummed, as these Cardinals really are Jumbo... ( about the same size as my Rummy's are ), I was going to order more, & of course they are out of the Jumbo's, & just have small ones left... at least I know they really are Jumbos, if they list them again...

Well, they are looking good, after work

Could have been Bag Buddies left over from days gone by. These do several things for shipping including a sedative for the fish. That would explain the fish on their sides and then perking up when put into new clean water. They turn the bag water blue.

These used to be made by Jungle Labs but were discontinued a decade ago. There is an Ebay seller offering them:
https://www.ebay.com/itm/171920419332

The ingredients include Methylene blue as well as a sedative an ammonia detoxifier and something else I do not recall. Meth. Blue oxygenates. It is one of the few things that can be used to revverse the effects of nitrite in a fish. If I remember right it helps turn the methemoglobin back into oxygen carrying hemoglobin.

well... 8 out of 12 of the fish made it through the night... I would have hoped for better, but being "jumbo" fish... being older, maybe that is an acceptable number???

the Cardinal eating Angel fish ( that ate all the previous ones I tried to introduce, that came in small ), was sure interested, maybe the matching colors, to the previous??? but it did not kill any of these jumbos... I sure like them, I'll be on the look out for more...

The blue stuff in the water can be Methylene Blue or any liquid medication that turns the water blue. Other medications can have ingredients like Malachite Green, Formaldehyde/ Formalin, etc.

The fish sedative used on jumbo fish is called MS222 and is used on large fish, not adult cardinal tetras. Large fish are things like 8-10 inch Oscar cichlids, 2 foot long arowana, etc. It is not used on jumbo sized cardinal tetras because they are simply large cardinals and are small fish.

You should not put fish into a tank with a fish that has a history of eating the species. If you really want to have cardinals with an angelfish that eats them, you should feed the angelfish really well before adding the new fish and keep the angelfish well fed from then on.

Your new fish should have been quarantined before being added to the main display aquarium just in case they have a disease.

A lot of companies in North America still use bag buddies. They used to sell them in enormous quantities even a retailer would take a long time getting though.

8 on 12 is a worry - that's a poor survival rate although at this time of year it might be more expected. The average 1 or 2 tank aquarium keeper can get by with no QT, as if they import a disease, as sad as it is, recovery/restart is easy. But with the stock your curiosity has led you too, you are playing Russian roulette. You like rare, expensive and often hard to replace fish, and with the quantities you're buying - you like to live dangerously, my friend.

Yes, I had best intentions of keeping a couple extra tanks, when I added 4 -10 gallons… one was always planned for cherry shrimp, and one for growing live foods… the other 2 for hospital and quarantine… getting those earth eaters, sent to me instead of Cupids, was bad for my plan, as those tanks got used to house unsocial fish… at least in the South American tetra tank, there are no rare or hard to get fish… the tank that worries me most, is the Hillstream tank, with rare fish, special conditions, and temperatures, they can’t just go into a typical tank for quarantine… I do think, having pristine water, and an over abundance of plants helps, but you don’t know, with the stress of shipping, but I am pretty picky about who I buy from…

That can work as long as they remain picky about who they buy from. I have had epidemics hit fish from the best of sources though. It just happens sometimes, without the seller knowing it. A few years ago I bought 50 cardinals from a trusted source, and they wiped out from what looked like flavibacter within a very short period. Meanwhile, he had several thousand and lost none. The fish can carry and control things but lose the thread when shipping stresses are added.
In that case, something probably went wrong along the way. Other buyers had no issues, but here, they came into contact with no other fish and went straight into a good QT tank. Stuff happens.
The pathogen was there, unseen, and might never have manifested itself until we added UPS-itis to the mix.

Would it have busted loose in their main tank, full of rare and expensive SA tetras? I'm glad I don't know.