Different but same cory.

[Tony ynoT]

These 2 are 2 of 8 that survive the last spawning and im wondering if they are 2 different strains or a defect in 1.

[Andy's Fish Den]

If they truly came from the same spawning, they are the same species. What happened is that one got more food basically. It happens all the time, I will have a batch of say 50 fry, there will be 5 that grow super fast, there will also be 5 that grow super slow and the rest will grow all about the same. 

[gardenman]

If you're talking about the size, it could also be a sexual difference. Male cory cats tend to be smaller and slimmer while female cory cats are larger and more robust. The one on the right may be showing longer fins and there are long fin versions of cory cats. If the parents didn't have long fins, it's probably a sign that they were culls from a longfin strain and the gene has popped back up in this generation. If you're breeding long fin cory cats you'll get some short fin ones mixed in.  Those get culled, but since they look like a "normal" cory they're just sold as normal cory's even though they have the genes of a long fin cory and could produce long fin cory fry.