I have a 45-gallon tank that's been set up for a year. No live rock or coral, only a few small fish at any given time, along with some sponge crabs & assorted urchins. Well, the inverts have thrived, but I can't keep fish alive! I use an Eheim wet/dry cannister and a CPR Bakpak skimmer, plus a powerhead for additional water movement. My ph is perfect, alkalinity is about 1.22-1.23. No ammonia, no nitrites, and nitrate varies between 20-45. I do a 50% water change every month. I have lost every fish I've put in this tank...most rarely last 3 weeks...even hardy fish like damsels and wrasses die. Mostly, fish seem fine for 2-4 days, are heavily breathing for another day or two, then die before a week is up. Could there be something in my water that's toxic to the fish, but not the crabs and urchins? Am I changing too much water too infrequently? I've kept fish for 30 years, and never had this kind of problem. Any suggestions? Thanks to anyone who can help with this mystery!