O..bummer. I'm not particularly fond of African cichlids....but I shall try my best to consoul you!
First off, I am envious of you being able to go to That Pet Place and get your fish! I currently have some fish on hold for me there...
...Crenicichla marmoratas to be exact! Wahoo..but i digress. Ahem. Its possible that you demasoni may have just passed away while you were gone, and the rams ate them/it. The reason i say this is cuz they ARE from different continents, and have different water requirements. It is possible that it just wasn't able to cope with the softer, more acidic water and passed away. You didn't include what the other tankmates are either, which could be rather helpful in solving the mystery. Rams to me don't really seem like cold blooded killers, but you never know. In a small enough tank, and if the demasoni isn't able to get away..it is possible. Have you checked on the ground around the tank? Sometimes fish in their flight to get away from attackers resort to jumpin.....sucks but it happens. Hope you have the best of luck wit your last one! Get him his own tank and then get some buddies for him. Then you will be able to watch them closer to find out whats going on! Side note, just how small are they? Bite size?