Water parameters can have a big affect on the conditions of fish. Ick is mostly cause by stress from a fish. When they're stressed they're not as strong against fighting diseases. Also if the cories were in a tank with other fish from ick, they could have been carrying it. I'm not sure that is the case since you didn't notice it on them. Ick free floats in your water (everyone can have this happen even if they have the same fish for years..it's just in the water) until it finds a host to attach to. If the fish is happy and strong than it is less likely to get it unless another fish in the same tank does happen to develop it. The ick stays under the clear coat so it is basically impossible to remove at this point. The best thing to do is to add salt and increase the temperature to around 83 to speed up the process. The time will vary, but once you notice the ick disappearing on the fish is when you need to start treatment. At this point the ick that was on the fish has reproduced and died off and the new ick is floating around trying to find something to attach to that it can live off of. This is the stage that the ick is suseptable to treatment. It takes a little time. You can do more research on it to find out how to treat it. Most places carry drops or tablets. I've always had great success from AP Quikcure drops. BEFORE you decide to treat it. Take time to do some more research to make sure it is ick. If you can get a picture of it on your fish posted here on the forum then that'd be a great way to find out. If not, try to find pics online that match what your fish's symtoms are. Good luck..Let us know