I actually had a power outage last night. Everything was off, and my filter was making a strange sound. Since I didn't know how long the filter had been off, I left it off for an extra 8 hours, so as not to release any toxins that had build up into the tank. I have a Fluval 403, does great on my 55 gal. I did a 50% water change after I got home and had dinner. Soon after, all 5 of my Lemon Tetras, started floating, and not moving water over their gills, they were dead. All my other fish are fat and happy, and still alive. They've been acting normal, and have been all day, so if anyone knows what may have happened, please, let me know. Also, I checked the ph, was between 6.6 and 6.4, and has been that way for quite a while, no nitrites, very little ammonia, about .25ppm. I've taken steps to rid the tank of ammonia.