I don't like saying this but rainbowfish are prone to Tuberculosis (TB), but it also depends on where they came from. If you got them from Gary Lange in America they would be fine, but from the UK, they might have it. Fish sold in Australia are regularly infected with it.
Typical symptoms associated with rainbowfish that have TB include ulcers, and when they are about to die, they bloat up like a balloon, stop eating, do a stringy white poop, breath heavily near a filter outlet or at the surface, and die within 24 hours of showing these symptoms. Hopefully yours don't have this problem.
Your fish should be old enough to breed and if you collect some eggs, you can hatch them and grow them up in the smaller tank and use the young fish to start the new colony. If you make a spawning mop out of acrylic yarn, you can pick the eggs off that and reduce the chance of transferring anything into the rearing tank.
Keep the adults in their tank and don't add any more fish just in case they have TB. Feed them last (after feeding any other tanks you have), and use separate gravel cleaners and buckets for that tank. If you only have 1 gravel cleaner, soak it in straight bleach for 30 minutes after using it in their tank, then rinse well with fresh water.
You can try loading the tank up with salt. I used to do that when my fish were doing funny things and it seemed to help. I added a lot more salt than I normally recommend and quite often the tank would be 1/3 to 1/2 sea water. I left the salt in the water for a couple of weeks and then diluted it out with small daily water changes. It killed the plants but usually fixed the fish.
2 of the 8 already look to be on the way to fish heaven 
