will some sort of zyme product help with decomposition of root matter as it dies, when a hermie is necked?(6 in same trough so all roots tangled i'll assume)
i'm a bit worried it'll start affecting the rest if left untreated.

I think that 'Cannazyme' is the stuff that you're reffering to. I think that ppl have used it throughout their grows, so it is a distinct possibility that you could add it to your res for that purpose, I reckon. Wait for an expert's opinion, though.

Edit: What hydro system are you growing in, anyway?

got 3" cubes on 6" cubes(rockwool-both)on 3" of clay pebbles on spreader mat,in toptray(with extra drainage holes drilled) fed via 2l/h drippers.(res is sorta acting like a bubbler cos of extra holes drilled allowing roots through). The f*cking things are over 5ft!!(4ft if they were on floor)and PLENTY of flowering sites(more than leaves!!)

The best thing in a nft system to do when taking plants out , if cut the 3 inch block out off the removed plant then use cannazyme in the res this will remove all dead roots ,just remember to check that the pumps do not get blocked up .

Hope this helps