Edit: It's been a week since my fish went from healthy to dying overnight. I believe my tank is now back to normal and all my fish are healthy. 1 fish died but the others lived. I'm still not sure what caused it; the tank was stable for over 6 months. My LFS tested my water and suggested my ph was a little high, 8.5, so I did slow water changes until I got it below 8.0. I also suspect it was stray voltage that might have caused the problems as I had just replaced a powerhead. Out of paranoia I unplugged everything except for the filter and one powerhead. I did plug in the skimmer for a couple of hours during the day, though. I finally did test the water and was getting readings of 7-14 volts so I bought a grounding probe. I don't know if it was high ph, stray voltage, or something else that caused this but I hope it doesn't happen again! Things I did: Wednesday night - replaced a powerhead (maxjiet 600 to hydor koralia 550) Thursday afternoon - noticed fish had rapid breathing, did a 25% water change. Thursday evening flame hawkish died. Friday afternoon - fish are still breathing hard but look a little better, added fresh carbon. Friday night I unplugged everything and only left the powerfilter on and 1 powerhead. Saturday, fish looking better did another 25% water change over time to slowly lower ph Monday, fish back to normal, tank back to normal, added grounding probe Hello all, I come to you guys for help as I'm at a loss. I've lost 1 fish and 3 others are in danger. Here's some background: 30 gallon saltwater aquarium FOWLR. Been running for over a year, has been stable for a long time with no sign of disease. 1 Flame Hawkfish, died a few hours ago... 1 Bicolor Angelfish, breathing rapidly, not moving 1 Bicolor Blenny, breathing rapidly, hasn't come out of his hole 1 Yellowtail damsel breathing rapidly, staying in his cave 2x Clownfish healthy and normal Paramters that I can test: Nitrates 10ppm Nitrites 0 Ammonia 0 PH 8.0 Salinity 1.024 Temp. 77f Filtration: Aqueon power filter Aquac Remora skimmer 2 powerheads 25 pounds of live rock 2~inches of live sand Wednesday night fish were fine and healthy. The next day when I saw them in the morning I noticed my angelfish was breathing hard, I was worried but had to go to work. When I got home in the afternoon I noticed all the fish except for the clowns were breathing rapidly and not moving much. I tested the water and everything was great. I did a 25% water change (about 7 gallons) and decided to wait it out. My hawkfish died an hour later. It's now 2:30 a.m. and the angelfish and blenny are looking bad. I see no obvious sign of disease and can't figure out what's wrong. There were a few things I did change before this. Wednsenday night I replaced a powerhead (maxijet to a hydor koralia) and added some truelumen lunar lights. I left the lunar lights on all night if that could affect the fish. The only things that I could think of that I have changed and could have caused this are: Lunar lights? When I was deciding on a position of the new powerhead It moved a bit of the sand on the surface, nothing major but anything's possible. Something toxic in the powerhead? I did rinse it before putting it in, though not very thoroughly. Stray voltage from the new powerhead? It's off right now. Those are the only things that have changed. It's strange that the clownfish and crabs/snails are fine but the other fish are dying. I'm debating whether I should do more water changes I hope I can save my fish but I would like to know why my fish went from healthy to dying overnight. Any help would be greatly appreciated!