Oh zoe, so sorry about your fish, my guess if not stiring up the bottom and causing a toxic reaction would be disease of some pathogen, bacteria etc. The ice just melted off my pond and for the first time I have 3 fish that did not survive, I know how you feel. If the ones you removed are still alive, give them a good salt bath.....also go over to koivet.com and ask your questions, they have a section just for sick fish and are very helpful and knowledgable