Diatoms will last until they don't. Just pesky things that you have to live through. High nitrate is almost certainly the rock. Denitrification happens by bacteria deep in the pores of the rock. Yours didn't have any. The pores are also full of dead bacterial mulm, which will delay the colonization of new bacteria.
I agree that adding the zeo nutrients should be held off until you need them. The zeostart is the carbon source, I'd play with doses of that, as most use half recommended. Maybe increase slightly? Overall, 5ppm isn't bad. 0 is a good target, but it's going to take time. I'd start worrying after the first year, until then, it's a young tank and needs to develop.
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Brad
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