Neon tetras are susceptible to Neon disease, a deadly one that has no cure. Cardinal tetras are not. Cardinal tetras grow, with proper conditions, to be slightly larger than Neons. If you are a beginner, and you have a 20 liter tank to start with (not a 2 liter goldfish bowl!), adjust the water, add some java fern or java moss or amazon sword plant, and add 5-8 Cardinals. That's what I did, and they are excellent! Good luck!
I have kept cardinals on and off for many years with reasonable success, but have found them to be a very shy fish shoaling in a back corner away from view. You can imagine how disappointing this is as the appeal of this little fish is seeing a great shoal of red and blue moving happily about the tank. After a bit of research on the net I used black gravel as a substrate in my new setup. What a difference! I have fifteen of these little guys and they seem to be perfectly happy and uninhibited, I even have fewer plants then I usually do. I should mention that I am yet to buy any bigger fish such as Angels and this may be a contributing factor in this newfound behaviour. Even with this in mind I believe that a dark substrate is important if you want to see cardinals at their best. Their colours even seem to be more intense against the black gravel!
I tried putting some cardinals in with 4 baby angels. One by one, the cardinals turned up dead. I added two more, and returned to the tank after having shut off room and tank lights. Then, I saw the 4 nickel-sized angels surround one cardinal, strike a few times, and leave. The cardinal died. They did not eat it. I would not have known if I had not "sneaked back" for a look. Apparently, angel fish eat neons and cardinals in the wild.
I have had a 280 L community tank for approximately seven years. Well established tank now, heavily planted, dominant plant Sagitaria. I tried introducing Cardinal Tetras a few times, they seemed to be okay, but died off within a week. I had an idea about the next time I would try. They are usually starved by the time you buy one, so I used a plastic container (like a potato salad container you get at grocery deli), and placed the fish in there to float them instead of the bag you purchase them in. I used freeze dried bloodworms and fed them before I placed them in the tank. They ate like crazy. I started adding the tank water to acclimate them. Then placed them in. Didn't lose one. All still doing fine, all thirteen of them. I have used this method on any new resident I buy. Have three tanks going, and this method seemed to work for all. Hope this little hint will help anyone who tries it.
In my experience with cardinals I have learned that they do not take shipment very well. With every cardinal purchase I have ever made, about two thirds of them do not make it through the first 48 hours. Therefore, I always account for this by buying at least 10 at a time, to make sure that I will hopefully have three or four to school together for the duration of their quarantine. I do highly suggest to have a quarantine tank available for them, and make SURE that the water parameters are dead on, that it has been pre-cycled, that the water has aged 24 hours, and that you have a bottle of Quick Cure on hand! After the shaky first few days, they should become robust and durable.
I finally got a large school (20) of these. Been looking for quite some time. They usually look pretty bad in the stores where I live, and they are usually expensive, but a large group came in and they were on sale. They looked very healthy, so I bought them. The store had them in 15°dGH and pH off the scale (basic). This was a problem, so I put them in the tank, still in the bag and, with a turkey baster, I SLOWLY acclimated them to my water parameters. Took nearly half a day. Then I poured them into a tupperware with hole in the bottom, and flushed them with tank water (about 4 liters). This may seem traumatic, but it's worth it to get out most of the diseases and parasites which inhabit all dealers' tanks. Then I put them in. I quarantined them for 4 weeks and brought that water chemistry to par with my main tank. Of the 23 that I bought (he gave me 3 extra because they are sensitive), 1 was dead on arrival, and 2 died in the acclimation process. They had sunken bellies, so they were on their way out anyway. So now I have 20 in a 280 liter tank, and no more deaths. Good fish...