Cardinal Tetra Diseases
Like all animals, your tetras can get sick. Here are some common ailments:
Noticing red or purple gills (obviously besides the red stripe that is supposed to be there)? Are your fish very close to the surface gasping for air? Ammonia poisoning is common in new aquariums that have not been cycled. A quick fix is an ammonia detoxifier like Amquel+ or Ammo-Lock (I prefer Ammo-Lock). If it's an emergency and you don't have that, do an immediate partial water change.
Noticing bloated tetras with raised scales? This is the result of a bacterial infection, often stemming from malnutrition. You will need to do a partial water change, and possibly get medication from your local fish store if it's not too late.
Are your tetras kind of laying on the bottom of the tank with rotted fins? You'll need tetracycline, just make sure to remove any carbon filtration in your tank before adding it.
Do your fish have white spots on them, looking sort like someone just salted them? That's ich (pronounced 'ick'). Any pet store will have a variety of medication for treating this common ailment. It usually is caused by sudden temperature changes, stress, and bad water parameters.