Hey there GentleWilliam , i have to disagree with MrWeeds here, this looks more like a pH related issue. The symptoms you're seeing like the rust colored speckles and the burnt edges of the leafs as well as the yellowing in general might be caused by a pH that's too high or a pH fluctuation meaning those nutrients would be less well absorbed . My guess is that and a nutrient lock out because you're feeding a lot of nutrients and over time it locked out the plant. Try flushing once by watering until runoff then measuring 20% more water and rinsing the soil well. then start feeding again at the same level but without the fish mix, which raises your pH quite a lot. Hope this helps ! π