semi you wrote, "My thought is that the pH spikes that killed the fish also blocked nutrient uptake".

Check out this post "Deficiency In Essential Minerals", it has a lot of great pictures and information. I listed the two parts that I thought was the most important as far as our problem goes.

1. PH level is very important, most all the elements have optimum PH level for absorption. The plant will not assimilate the nutrient if the PH level is too low or too high. Keep PH between 6.8 to 7.0 anything under 6.6 can kill off your BIO Filter. Anything over 7.8 your roots wouldn't be able to absorb any of the nutrients.

2. Iron, manganese and zinc are three essential elements that become inaccessible when PH is above 7.8, Hence their may not be a deficiency of one of these minerals, if your PH levels are stopping your plants from using them.