How big do fingerlings need to be before being introduced to large tank with big fish?

[Michael Welber]

I have some fingerlings in a bucket in my large tank that also contains some big fish. How big do the fingerlings need to be before they can be introduced into the large tank (and not get eaten!)? Right now they are about two inches big.

[Vlad Jovanovic]

Big enough not to fit into the mouths of the larger fish. That's the general rule (as far as I'm aware) across the board for most all species...

[Michael Welber]

Hmm. OK. So I guess I should go to the tank and measure their mouths. 

[Keith Rowan]

what kind of fish?

[Vlad Jovanovic]

Well, I'm sure that you can probably just eye-ball it...but if you  are serious about wanting to measure...the formula for determining the "oral gape surface" is

Mouth width multiplied by Mouth depth, divided by Body width multiplied by Body depth:

Mw x Md

Bw x Bd

(for practical purposes you can interpret "Depth" in this case as "height")

The oral gape surface of a species (as determined by the formula above) will determine the size and/or the nature of the food they can capture.

But honestly, most people just eyeball it...

[Michael Welber]

Yes. I'll eyeball it. They are blue tilapia. Hate to lose any of the smaller fish.