Nope, it's not Delta's that are prone, it's Double Tails. You were probably mixed up with the abbreviations, DT is Double Tail and DeT is Delta. yeah it can be confusing, took me a while to figure it out too haha
The thing about the babies is that most of them are culls from a spawn from a breeder, so the first that naturally have genetic deformities and the ones that were stunted growth or they didn't think they were going to be good fish. But they make great pets! Fish with deformities usually can live a great life, that's why when breeders cull they don't just...well for lack of better term, kill them, they look for people who will give them good homes as pets. But PetCo saw this as well as other stores I'm sure and said "Hey! Let's make some money! Here, we'll take the babies and sell them for you and we both make money, sound a good deal?" and so that's where we get our babies from! Not all are deformed though and I'm sure your little one is going to grow up nice and big, especially with good care!
Anywho, that's my lecture on "Where Do Babies Come From?" lol and that's super adorable he was 'helping' you clean off the algae ^_^