What unit are you using?
Between 30 and 60 ppm would be fairly low, and not a problem. Between 30 and 60 degrees would be VERY high.
However, there are other water parameters that I would be more concerned about testing if I were trying to diagnose mysterious fish deaths. Have you checked pH, ammonia, and nitrite?
If you need help troubleshooting, we'll need to know a lot more details, such as:
- size of tank
- other fish in the tank
- how long has it been running
- any symptoms/visible indicators on the pleco before it died