Yes, of course, I do not believe in spontaneous generation. There must be a reason, and I have a hypothesis or two to explain where the fry came from.
But what do I tell the children?
Children who never saw a carrot develop from a seed before this year?
Children who never saw saw slugs before?
Children who are just starting to trust me when I tell them that the world that matters is the world that they can observe?
I'm not going to tell them anything. An empty tank now has a couple dozen fry, really just eyeballs with a tail attached, cute as can be and not likely to survive the week. I'll let the students figure it out.
My lambs crammed together around the tank, marveling at the tiny critters that started their lives right here in Room B362. They see what they see.
If I tell them what they're supposed to think I'll ruin the whole thing.
The fish fry photo taken from ExoticGolfish.net here.