Most gouramis are better than the dwarf, which are heavily inbred and regularly infested with Fish TB and the Gourami Iridovirus. The Iridovirus kills them over a week or two so it wasn't that. It was probably an internal problem like organ failure which was possibly caused by TB.
Don't add any new fish to the tank for at least a month.
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Do the following to reduce the pathogens in the aquarium.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
