Sorry for your loss, strip and remove everything from the tank, bleach it and rinse very well, would be good if you could leave it to dry out for the day, boil all the sponges, put some salt in a bucket of quite hot water them run the filter in the bucket, throw away gravel and replace it with new, clean and sterlise all plants and ornaments, set back up i ran it for a couple of days with some salt added to the tank, then when i added new fish, i removed the salt out of the water, and added some gravel and mature filter sponges from my fancy goldlfish tank.