Lots happened in this grow the last few days…
Dealt with some issues, solved – algae. Now running 3.7-ish ppm of calcium hypochlorite pool shock. Thanks @anon32470837 for calculating that for me 
I have also light-proofed the white PVC bulkhead fittings that were, seemingly, the epicentre of algae growth.
Reached in with my hand to feel for slime on all the usual spots – nothing, so that makes me pretty happy.
However, root development has slowed a bit. And they are still brown and limpy-looking. Plants above ground seem to be doing OK, growing, branching, etc.
Roots:
Plant:
When i did refill, I did not go with megacrop, but instead used my regular Green Planet mix, as per before, but with about 1.6EC including calmag. Next time i’m gonna mix a little lower. Back to square one, I hope. Will see how this goes, maybe next time try beneficials…