I agree, I don't test my water and I don't buffer my water. But then i've been doing this for a long time & follow the advice I provided earlier. The biggest thing years in the hobby bring you is the ability to recognize when there is a problem and what to do about it. With a new set of fish this can often be a problem. While you may be able to recognize potential problems with mbuna, plecs are a new game. Thus the learning goes on.I may have been amiss in not testing my water sooner, but - from experience - I've seen too many hobbyists get lost in testing, adding chemicals, and chasing their tails trying to get a balance. IMHO, a well-cycled tank beats chemicals anytime.
Its not so much the wrong tankmates as they could be perfectly compatibable given the right situation, its more the wrong fish in the wrong tank. 70L is pretty small as plecs go, especially for the likes of L190 which can push 14"+. Not given ample room and hiding places and problem can occur. This is almost contrary to mbunas where the more you seem to add the more dispersed the overall aggression in the tank is. Again, just a learning expereince.I also think the attack I witnessed tonight added strength to the other poster's theory about another pleco killing the others. Again, probably my mistake in not choosing the right tankmates.