Sounds like you might have a high level of copper or heavy metals.
Recently a fellow local club member had an almost complete wipe out because one of the heaters broke and copper leaked into the water column.
You are not testing for any of those, just you regular parameters.
The easiest way to find out is to use a poly filter pad. The changing color one, that will let you determine if you have any metal trace. Your other option is to do a onetime test with
AWT, a onetime test with cost you $40 and a 4 test package $110. (I think the later is a better deal and will help you with your future tests)
There is something that is poisoning your inhabitants. It is either a trace element that you are not testing for or some equipment that is leaching the poison. If this was the case, then you might have compromised the Live Rock and even the sand bed... and there is really only one option, to start from scratch.
Once we eliminate the heavy metal as the cause, or while you are determining that the other factor is Oxygen. Lack of Oxygen will kill your fish.
I imagine you have good water movement, and the movement is breaking the surface tension of the water so there is proper oxygen exchange. In some cases, a film develops on top of the water, and lack of proper water agitation will impede this exchange.
One of the easiest solutions is to introduce an air stone in your system. If you can run the air from a "fresher" source (ideally outside the house) you will improve the introduction of oxygen vs. CO2.
I will also look into the phosphate. A 0-5 reading can be deceiving. Algae consume those phosphates, and they are present but are being consumed. That means that you could be introducing them through your RODI water. Which reminds me.... you are using RODI water for your water changes right? And when you make the water changes you introduce water with the same exact parameters, as far as salinity, temp, approximate pH, etc... Right?
I would also test your newly made salt water for phosphate.... as a matter of fact, if you find the time, try to test your new saltwater for all the parameters for which you have a test kit.
Fish killers are ammonia, lack of oxygen, poison and sickness (parasites, health issues, etc..). Fish are less susceptible to parameters swings... they can actually survive larger temperature swings, and remain alive under different salinities. Corals are more delicate... but right now we are dealing with fish deaths.
Don't be discourage, Chingchai, who is building an amazing
1000 gal tank in Thailand, with top notch equipment, recently lost most of with fishes to oodinium. As you can see, extremely experience reefers, with fine tune setups, and years of knowledge on us suffer from these events as well. You can read pages 85 and 86 where the issue is discussed.
Remember that the cardinal rule of Reefing is patience, tons of it, as a matter of fact.
One last thing (I am writing them as they pop in my head so forgive me for the lack of order in the post) how about power loss?? You don't stay in front of your tank all the time, right?.... do you think it is at all possible that throughout the day your lights (electricity) go out and then return an hour later, stopping any water flow and therefore producing the lack of oxygen?
Do you have a backup, battery operated air stone, or power head that will remain on while this happens?
Good luck and keep posting, we will try to get you through it.
