ok here are the problems i viewed about your tank
The fish choice is very bad, you saying plecostomus probably means a common plecostomus, these require at least 75 gallons since they can reach up to 20 inches long :O I think the reason why your tetra is sick is because it is stressed, it doesnt have any of its own species to school with which will stress it, even to the point of death. It wont school with the other tetras since they arent the same.
You said you use a CO2 unit with no live plants, that is a waste of money as the fish dont use CO2, plants do, also excess CO2 in the tank is no good for the fish
Never give your tank direct sunlight at any time of the day, indirect is ok to an extent but if the sun is lighting the tank, then it is very bad
Last but not least, your water parameters, i need the exact result of it before i can create a successful diagnoses. Just saying it was good actually gives me the feeling that is was bad since either you are hiding bad water params (i hope not) or your lfs is lying, what test kit do you use?
My diagnoses, The fish is highly stressed from a combination of bad water params, over stocking and not enough of its own species.
My treatment, find homes for all the fish, you can keep the sick one if you like but the plec and the shark should go, then start over with a new stocking scheme, also i dont know why i forgot about this but can you give us a picture