They need to school. Get something else that doesn't mind being in singles or in small numbers. If you just keep 2, they'll be stressed out, prone to illness, and you might lose them.
Like I said, I'm not sure if the heater is old. If it works, then it works. If it doesn't, you might get a jolt when you stick your hand in the water. The way the old ones worked, there was a little light and you'd turn the heater on until the water reached the temp you wanted and then you'd dial down until the light flickered and that would set the temp.
Did the person you got the tank from say it worked? It doesn't look submersible so just make sure and keep the top above the waterline. If it has any info on it at all maybe you can google it and find out what kind it is and such.