There is an infinite puzzle brought up here that revolves around lighting. The reason I say infinite, is that there are infinite combinations of bulb brands vs age vs life vs ballast brands. The same bulb on different ballasts is driven in different fasions to produce sometimes a really bad spectrum of light. Old bulbs are a guarenteed recipe for disaster. You don't have any idea if they are intensly ultraviolet or way up in the 4,000k range on a used ballast unless you have the technology to test all that. Handling, the way the bulb was used, orientation (verticle vs horizontal vs ratings) fans blowing right at the bulbs, all cause undesired shifts in the spectrum. Most often we assume the wave length is what the bulb is rated for under all conditions and that is just simply not true. If you knew where a bulb came from, who used it and for how long, then a bulb that is a little used can be worth buying. Otherwise, its best not to use such a thing. A day or even a couple of no MH days with only some actinics on should not hurt anything.