How do you know when a mosquito fish is dead?

when the gills stop breathing or when you poke it and it stops moving and some fishes float to the bottom when they first die and then they''ll start to float up because they release gas .

Main signs that your fish will be dead is if there is no movement and it is being carried along by the water current. Also check for any visible signs of disease. If you cannot tell a classic sign will be using a net to gently move towards or possible touch the fish and if there is no reaction your fish is likely to be dead, generally your fish wont float at the surface unless sufferin from diseases such as dropsy.

If it is discovered that your fish isnt dead check for disease and odd behaviour. It is also possible that your fish is resting as throughout the day your fish will rest at the bottom of the tank.