Be very concerned! They eat a lot, and saying that doesn't really reflect the reality. They eat a lot. A fuge might help you out in a pinch if you have one or two cuttles, but I wouldn't rely on it for any length of time.
You could buy a big rubber maid container, 20 or 30 gallons, and throw a heater and air pump on it to store your mysids. The larger volume will give them more space to cut down or predations. Adds some plastic screen for more surface areas as well, and don't for get to feed the mysids too. Sadly, the larger the container for holding mysids means it becomes harder to see and catch them.