I have kept cardinals in a 90 L Jebo style tank for 2 years. They are some of the most beautiful fish I own. Tank bred stock will generally be more adaptable. They will normally have grown up in pH 7.0, harder water than wild fish and be accustomed to this pH and hardness. Avoid wild stock, unless you are an avid aquarist. They thrive generally on good quality flake. Some bloodworms and brine shrimp serve well as a weekly treat. You can never spend enough on QUALITY flake and frozen food for that matter. Flake is usually the staple diet so do not be tight. It will cost you more in stock in the long run if you skimp and your fish will not look as good. Try and find a flake that has freeze dried bloodworms and daphnia and smells strong an fishy (I use Hikari GVG mix). As with most tropical fish, cardinals love fresh water and a regular water change is vital.
Recently I have introduced 6 Cardinals to my tank consist of 10 Neons, 10 Pristellas and 7 Black Neon and they seems to be blend well in environment with plenty of vegetation. From my observation, they are not as timid as the neons, but they tend to be slower, especially during feeding time. The full bright blue stripes distinctively differentiate them from the neons. They are peaceful fish which should be kept with similar size tankmates.
To better the survival rate of delicate cardinal tetras, I wait until the day before the shop's weekly shipments come in to pick them up. In this case I purchase them on Wednesdays. This will have given them a week to acclimate and settle into their pet store tanks and enough time to weed out the weaker ones. Remember that susceptible fish are highly stressed by the process of transport and then transfer to new water in the pet store. Then if you buy them immediately after their arrival to put in your home tank, this could deal the final blow which will kill the fish within hours or days. I learned this the hard way. If there is a high demand for cardinals in your area, see if the store will let you buy them in advance and hold them for you until they have adjusted. Remember that fish sellers' stocks of Paracheirodon axelrodi are virtually all wild-caught, so the longer you can keep your cardinal, the better. Because of high-demand and low survival rates, the trade could threaten or endanger this species--not to mention the human-engineered threats to their natural habitat in the Amazon. I hope to succeed in breeding and rearing this fish someday.
I've never had a problem with these fish, only neon tetras. One cardinal tetra survived pH 8, moves from a 100 L tank to a 200 L, a mysterious disease outbreak, then a move to a 150 L before dying. I would just like to add that I have had two experiences of these fish playing dead when they are being moved. One almost got flushed when he was floating upside down and not breathing. He was pretty convicing. He is still alive and happy today. Wierd...
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