Single yellow labs

I have had one electric yellow living with various other malawi haps/peacocks with no problems.

You can keep them by themselves but their behaviour is much more interesting in groups of 4 or more.

What size was the yellow, and what size are your other fish?

Sounds like he was bullied to death...

The tank has fish from different areas of the world with different water needs. The t-bar cichild (Sajica) is Central American, for example, and needs neutral-ish pH. Your lab, a mbuna, is from Africa, and while people do mix them, it's not an ideal situation. The labs need high pH, much higher than your Central American finds ideal. You proabbly should decide whether or not you want an African cichlid tank or a Central American cichlid tank and stock as approriate for that choice. With what stock you have now, I personally would not add Africans . . .