Same thing happened to me about 6 months ago. I lost all my euphylias except one green/green frogspawn which hung on and a purple tipped torch that was totally unphased. Heads were popping off left and right. No rhyme or reason to it, nutrients were almost identical to yours. Ironically I’ve stopped chasing the numbers, my NO3 and PO4 are higher than they were and the remaining euphylias seem to be bouncing back. I have no idea what the problem was, I chalked it up to brown jelly. Had a couple awesome pieces, a teal hammer, orange/pink FS and a huge purple tipped FS all kick the bucket, totally blows