Continuous wave (CW) operation of erbium 3+ (Er 3+)-, holmium 3+ (Ho 3−)- or thulium 3+ (Tm 3+)-doped ZBLAN fibre lasers, emitting at 2.7, 2.9 and 2.3 μm, is based on the depletion of their lower laser level by excited state absorption (ESA) of the appropriate selected pump wavelengths. This additional absorption process limits the efficiency of these laser transitions. The problem can be solved by the proper co-doping of the laser active dopants with other rare earth elements. It is shown that the desired depletion of the lower laser levels can be achieved in the case of Er 3+ and Ho 3+ by co-doping with Pr 3+ and Eu 3+.