@14g Nano
From a troubleshooting perspective:
1. It's likely nothing to do with your calcium dosing.... This generally wont affect fish - your invertebrates can be sensitive to alkalinity and pH. If there was something very off, it will likely impact you inverts first. It's simple, if you're Alk/Ca/Mg levels are dropping, you should dose. Many invertebrates and bacteria will use these. The microfauna are important too.
2. It's probably not a toxin (spray, cleaner, etc) for the same reason as #1.
3. It wasn't an ammonia spike, for the same reason as #1.
4. I can't think of any disease that kills overnight. Maybe velvet, but generally you will see signs of this (or any other disease) before an animal dies. With an untrained eye, it could perceivably go unnoticed. Even if you don't see the ailment though, you will notice your fish swimming 'weirdly', loss of appetite, skinny stomache, etc. Now, if it's been a few days since you gave the fish a good inspection, things could have escalated to the point the fish died, without you noticing....
5. Like
@ca1ore mentioned, my prime suspect would have been a loss of oxygen. In the event of a power outage, your fish will die before anything else, because they require a higher oxygenation level than your inverts do. That being said, it doesn't look like you had a lot of fish in there, so I would guess the fish would have lasted at least 6-8 hours before they got into trouble. Perhaps if the oxygen levels were low to begin with, a power outage could have a quicker impact. Someone mentioned surface agitation. Aiming a powerhead so that it causes the water at the surface to ripple, can help achieve this.
6. Based on the pictures, it doesn't look like you have any massive bacterial growths... Overdosing on an organic carbon source can a bacterial bloom, which can lower oxygen levels, but I didn't see any mention of organic carbon addition.
Recommendations:
1. Get a powerhead (or 2) for your cube. it will provide flow which will aid in filtration, waste processing, oxygenation, and will be necessary when you undoubtedly decide to add some corals. A small Sicce, maxi-jet, Seio, etc can all be purchased for < $50.
2. Consider quarantining your fish before you add them. The necessary supplies can be had at an affordable price-point, and will save you many headaches down the road. Even if you don't treat the fish prophylactically, you can observe before adding to the tank. I prefer prophylactic treatment, but, observation is better than nothing. Visit the fish disease forum for details. The ONLY proven remedy for Ich is hyposalinity and copper - anything else you will be wasting your money on.
3. Lastly, get to know some of the Reef Squad team, along with other prominent and knowledgeable members of the R2R community. These will be your 'go-to' peeps when you're in trouble. They will help you avoid wasting time sorting through the myriad of (well-inteded, but misguided) recommendations, and be your quickest path to success.
Keep your chin up. Keep studying, reading, and learning, and things will get better.
Nathan
