I know im a few years late but want to prevent others from taking any advice in this thread, what a shambles.
NEVER mix meds NEVER!!!!!!!!
NEVER euthanise a fish with cold water and ice!!!! NEVER!!! it takes a while to die and how do you think a fish will feel slowly turning into an ice cube? the cells form tiny icicles - its a very painful death.
best way is to slowly add clove oil, small ammounts will sedate, so once sedated you can slowly add more clove and this will humanley euthanise the poor buggers, leave the fish in the water container you use to euthanise it in for a few hours to make sure it has worked.
With all the forums and info on the net i find it frustrating that you went into panic mode changing one med for another and mixing! a little googling would have told you that this stresses and makes fish worse. 3 days was not enough time to let it work, you should of run the course and if no better you start it again..myxazin is much better than interpet, and melafix should of been used wen myxazine had done its job with the fish fully improved, being added a day or 2 after myxazine finished and carbon run to remove whats left, then the melafix then works to make sure illness dont return (so long as water chemistry fine and water kept clean)