Hmmmmm... Interesting thread, almost like a detective novel...
I read that you have an algae outbreak, how bad is it, and do you have macroalgae as well? If you have a big filamentatious algal outbreak and macros, keep in mind that during the night when the lights are not providing energy, the plants that normally produce O2 are now consuming oxygen at prodigious rates. If your circulation is not vigorous, and your temp is up, then your dissolved oxygen levels will drop low enough to stress, even kill you fishes. The most active fishes will be affected first, followed by the more sedentary fishes depending on their rates of oxygen consumption. Blennies, dragonets, and other inactive fishes will be the last to succumb to oxygen-related stresses. This drop in oxygen levels may not actually kill the fish, although it
will allow opportunistic infections (like ick) to gain the upper hand in the constant battle with fish immunology. O2 deficiencies lead to decreased appetite and lower energy states, with corresponding drops in metabolic rates. All these then start a cascade of events that compound each other, leading to decreases in the fishes' immune status to pathogens that normally might not even affect them. I would be interested to see how low your O2 levels drop during the night. You might try to borrow an oxygen sensor (Pinpoint makes one, I believe they are around the $250 US price range)
In the ocean, you occasionally will see eutropic effects (in areas where there is poor current and water exchange) on fishes on a large scale, especially in bays and shallow inlets where solar heating during the day and heavy organic loads lead to heavy algal growth and elevated nocturnal water temperatures. The oxygen levels in such bays can sometimes drop to near zero, leading to mass fish kills or to outbreaks of opportunistic infections in populations of fishes in these environments. You would think that this would be especially true in estuary areas, but tidal flushing along with the influx of fresh (no salt) water seems to make this less of a problem in these habitats. You might hit up Chris or Jerel for more info on this, but I would think that this is the crux of your problem.
Getting the O2 levels up at night, by removing sources of DOC's that can be reduced by oxygen (and thus consuming oxygen), removing as much of the algae as possible, switching to RO/DI water to reduce the nutrients available to the algae, skimming 24/7 for a while (or at least at night, this will remove the DOC's and oxygenate your water), running a
reverse photoperiod refugium in your sump (helps to export excess nutrients as well as stabilize your pH and increase your nocturnal dissolved oxygen levels) and increasing your surface and deep current will help mitigate some of these causes for both algal growth and decreased nocturnal O2. It might help to reduce your temp some (I don't know where you are on this, but staying no higher than 80F for a while will keep your oxygen saturation levels higher than those at 84F, remember, higher temps = lower O2 saturation) I normally recommend higher temps for the reefs*, but not while there is a possible oxygen level problem.
I hope that this is at least something to go on now, Everyone here has made good recommendations on the issues brought up. In tanks with heavy algal growth, many nutrients are locked up in the algae, as well as their wasteful consumption of nutrients that could otherwise be used in creatures/organisms that we WANT to cultivate. It is very difficult to rely on water testing to see what the problems are when the nutrients are locked up in the algal biomass. If you have algal biomass, then you have water problems, and switching to RO/DI will at least remove the nutrient source (tap water). Might check this site:<
http://www.waterdata.com/ > to see if your municipality has it's water quality listed. surprisingly enough, some local water supplies provide a large amount of nitrate as well as phosphate for algal growth.
* Interestingly enough, temps in the Indonesian oceans vary based on seasonal upwelling from 82F to 85F, as opposed to the waters around Hawaii, where the temps vary from 78F to 82 F, but then again, that is another discussion (heh heh heh!!!)
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