Hopefully if you have some plants, it will shade the light a lil, and if the light is not too bright, like a LED color light, should be ok..but if it's a real bright light, and if you don't have enough plants to shield some of that light.. I am not sure if it will hurt your fishy..but I am pretty sure, it may stress him out some.. try shutting it off as soon as you get back..or if possible have someone do it for you
