
WELCOME TO FISHLORE!!
well 1st of all... ure going to need a much bigger tank than what u have. these "sharks" aka catfish can grow up to 24" (thats 2 feet!) yet they generally stay at about 12-14" in a large aquarium. one of these boogers needs a 55-75 gallon tank just to himself.. so by having 2 you need more room.
are these tetra pleco wafers small?.. usually catfish are known for eating algae hence why i feed my catfish an algae wafer (kindof large sized) and a bottomfeeder wafer (small wafer) i have a lot of fish that eat the same stuff so there is no waste
the more u feed fish the more they grow.. just keep that in mind.
columbians are never the best fish for a beginner because they go from freshwater to brackish to complete saltwater in their lives. which limits what u can put in with them. if they dont get this change in their water they can have a very lethal skin condition develop.
if uve only had them for about a week im sure u can return them if u decide that they are too much work. but remember that u need to buy a huge tank asap to properly keep them. if i were u i would switch to a redtail shark or a rainbow shark. they are very easy to keep, much more exciting to watch, and u can put other fish in with them. its just an idea.
good luck to ya and your fishies
edit: BTW columbian sharks have their own section in the forum. to get more answers to ur questions its best to put them where they should be so people can find them to read.