well, your water has some problems. how long was the tank itself set up? looks like you are still cycling, which means there are toxins like ammonia and nitrite in your water which can harm your fish. while your ammonia is at a somewhat okay low level, those nitrites are waay too high and are probably what caused the death of your fish. during a cycle you should do water changes to keep those levels under .25 every day. nitrites of 1.5 can be a death sentence.
to keep your other plec safe, start doing water changes every day to keep those levels for ammonia and nitrites down. as long as you put in water that is the same temp as the water in the tank, and use dechlorinator, water cahnges will not hurt the fish. try doing them at 50%, if that doesn't bring the levels down to below .25, do another and another. that is the only way to keep your fish healthy at this time.