You might be dealing with the Gourami specific Iridovirus, aka Dwarf Gourami disease.
If so there is no known cure.
I would doubt it’s the DG virus only because it isn’t known to infect other gourami species to the best of my understanding.
But that may well be changing as the viral cause has been spread world wide by the aquarium trade. In the first decade of this century around 22% of DGs tested on the fish farms in SE Asia were found to be infected and it has been shown that the virus can be spread through water.
Quote “The ornamental fish trade is suspected of facilitating the spread of an emerging viral disease.
The virus, which affects the Dwarf gourami, Colisa lalia, is so similar to one that infects farmed Murray cod, Maccullochella peelii peelii, that the two are believed to be a single species with a common geographic origin.
A team of Australian scientists, headed by Professor Richard Whittington of the University of Sydney, sequenced the genes of a virus that killed Murray cod in a disease outbreak in 2003 and compared the sequence to that of viral genes from imported Asian Dwarf gouramies, that had died in Australian aquarium shops.
When the sequences of the two viruses were compared, they had almost completely homology over 4527 base pairs with 99.95% of the sequences identical.
The two viruses, which are known as Murray cod iridovirus (MCIV) and Dwarf gourami iridovirus (DGIV) shared remarkable similarities - 99.9% - with a third virus known as infectious spleen and kidney necrosis virus (ISKNV).
All three viruses are believed to represent a single species within the Megalocytivirus genus and are thought to have originated from the same original location.”Unquote
Aquarium trade may have spread gourami virus - Practical Fishkeeping
It’s possible the sparkling gouramis have something like a mycobacterial infection. Mycobacteria are contagious & eating a tankmate that died is an excellent way to pick up this infection. Snails & shrimp aren’t the only critters that scavenge deceased fish.
