Nitrates do not directly kill fish. All the studies show that nitrate in and of itself is not toxic to fish unless the levels are extremely high (far greater than you would ever get in your tank). That's what the studies show but I know tanks with really high nitrates tend to have much higher losses than tanks with lower nitrate levels. Usually high nitrate indicate poor overall water quality so perhaps poor overall water quality is responsible for the higher losses and not just the nitrate level.

Crushed coral is just the wrong size media to be using with a dsb, with or without a plenum. You should be using a finer substrate, either fine sand or the special grade aragonite made by carib sea. People have had good results with both. IME, tanks with crushed coral tend to have more problems with nitrate, phosphate etc. Probably because so much detritus can get trapped in the larger substrate and create a nutrient sink.

It would take ALOT of macroalgae to bring nitrates down from "off the chart".