disappeared

Have been off the forum for a long time, just busy with the stuff of life (including fish)....however I am currently experiencing a "Star Trek transporter" effect with my gourami....he's gone. 20 gallon tank (UK), six copper harlequins, seven neons; a few cherry shrimp, a lot of small pond...

Normally have five fish in the coldwater tank. Two goldfish, one zebra style fish, and two minnow type of fish. Now the smallest minnow is GONE. Completely disappeared. I have checked the tank several times, including with a light and with a stick to prod through the two small fake plants...