I might be biased by the articles I was just reading, on night time oxygen concentration, but this sounds just like it (Reefkeeping...3 parts...by
Eric Borneman). Clownfish and gobies etc can tolerate very anoxic conditions , but swimmers like Angels and chromis ( Tangs, firefish) etc need close to saturation. You didn't note any skimmer or other airation.....so it is possible that your night pO2 is getting low and stressing higher demand fish. This could be observed with an ORP probe or deduced by diural fluxuations in pH.
The test tank was a skimmerless 10gal (witrh clownfish)...pO2 dropped to 16% at night from 90+% day (lights on) concentration. Try an airstone (in the refuge on a timer??)) at night and see if your ph stablizes (or ORP).Ohhh ya splurge on the Salifet alk kit and maybe throw in a calcium one too.
My 2 cents....good luck