I have some baby Canas that are a fraction of the size of their bigger brothers and sisters. Try 1/20th the size!!! Where they're about the size of the last digit of your pinkie, their brothers and sisters are bigger in diameter than if you touched your thumb to your middle finger.
I didn't want to set up another tank (or Rubbermaid container, as the case may be), but I've been wanting a way to separate the runts and make sure they had access to all the food they could ever want, and I also wanted to make sure they didn't have to compete for it.
Here's the solution I came up with ...

I bought a Sterilite container from Wal-Mart. Cost was $1.37+ tax. I had a suction cup at home and put it through one of the holes and attached the container to the inside of my aquarium. It's right at the top of my tank, right at water level. Then I took 16 of the runts from the nursery tank and put them into my new handy, dandy "runt-grow-out container" and added a plentiful supply of peas and collar greens.
Cleaning is easy. I just pour a little water into the container and it flushes everything out the holes.
I'll get pictures later today, but it takes up very little room in the tank, and doesn't interfere with my putting the top on the tank. It's a cheaper and more flexible solution than the nursery nets sold in the pet stores.