Mbunas dying. Can someone help?

[BellVel]

Hi, I'm new to the hobbie, and two months ago I received a new aquarium (160L/42 gallon) from a friend. The aquarium had a mbuna, a small koi and a pleco. I separeted the koi, and after some time introduce more 6 mbunas. But the last weeks they started dying all of the sudden. I thought It was the pleco since it's very big (25 cm/10 inches), and started feeding it more. Still, they kept dying. Can someone help me? I dont know what to do, they dont show any signs of disease and water conditions are fine.

pH: 8
Amonia and nitrate: zero
GH: 8
Temp: 28°C/ 82°F

 

[Ulrichsd]

How is it going over the last week? How long did you cycle the tank for and what kind of test kit are you using for your parameters?

 

[BellVel]

How is it going over the last week? How long did you cycle the tank for and what kind of teddy kit are you using for your parameters?

So, the remaining fish were just fine for the past weeks, until today that I found anothee one dead. I took pictures this time. The others seems fine.
So, as I received the aquarium full from my friend, I just transferred the water that was already there (for about six months), I did not restart the cycling. Even so, I introduced Microbe-lift Special Blend during the first week.

i use labcon Teste for GH, NO2, pH and ammonia. Other than that I use seachem Malawi Victoria Buffer, Seachem Replenish and Stability and Labcon Protect plus.