what kind of gourami is it? have you changed anything in the tank (i.e. has it finished the cycle with fish, have you increased or decreased the temperature, moved any decorations around, etc)? i had a
betta with a few corycats and when i replaced my heater and got one that actually worked and kept the temp at a steady 78 she decided that she felt good enough to attack them

! luckily i had a 10 gal i could move them into.
also, how many fish and what types are in the tank? is this the 20 in your aquarium info?