you are mixing colour temps
a flash is (somewhat) daylight balanced
and the light in your tank is not daylight balanced
hence the loach is (somewhat) of a proper colour and the background isn't.
some ways around this:
-over ride the camera and under expose the background so it dark.
-or don't shoot with flash and set the camera to whatever the colour temp is for the tank.
-or bathe the whole scene in flash and set the camera to daylight balance. This would involve multi flashes...best to avoid.
-or don't worry about it
