[i have crabs]

exactly what mr alberta said some nudibranches releases toxins when stressed or dieing try to find out what kind you had on the net and look for shure,but in a nano a 25% water change 2 or 3 times over the next week if thing do start lookin better carbon in the filter will help tones
i also dont think 1 clown and 1 damsil are pushing the bioload of a tank that size but i still wouldnt be adding anymore.