Erbium-ytterbium co-doped phospho-silicate double-clad fibers are used in many applications were powerful 1.5 μm sources are needed, such as telecommunication systems, LIDAR, medical lasers and much more. These fibers are typically pumped with diodes emitting at 915, 940 or 976nm to excite Ytterbium ions, which in turn transfer their energy to erbium ions through a phonon-assisted mechanism, thus leading to 1.5 μm emission. This energy transfer requires a large phosphorous content in the core of the fiber and therefore these fibers exhibit typically high numerical apertures. Properly optimized, the ytterbium to erbium ratio will minimize parasitic emission at 1 μm which provokes system failures through non-controlled spurious laser effects. We have recently observed, on such optimized fibers exhibiting 12 μm core diameter and 0.20 numerical aperture, that long term operation in CW mode in both amplifier and laser configuration, leads to a slow and irreversible decrease of the output power. This phenomenon starts at moderate signal power of just 7W and increases rapidly with the output power. This phenomenon is also observed in polarization maintaining version of the very same fibers. We have studied this phenomenon which resembles the well-known photodarkening effect in Ytterbium doped fibers. Our experiments show that all the commercially available fibers tested exhibit the same behavior. We will present a tentative explanation of the phenomenon and some solutions we implemented to drastically stabilize the output powers up to 20W enabling the use of such fibers in many industrials applications.