Comparison of the output characteristics of different erbium doped fiber lasers show that the threshold pump power increases (by a factor of 4.6) and the conversion efficiency decreases (by a factor of 1.5) as the erbium concentration is increased from around 150 to 1040 mole ppm Er2O3. We propose that these two effects are mostly due to rapid interaction (perhaps upconversion) between a subset of paired ions. This work suggests that for Al-Ge- doped silica fibers, concentrations of 150 mole ppm or less should be used for optimum output power. A fiber with this concentration produced a low-threshold laser with a total power conversion efficiency of 90.4%.