on my last attempt, I placed the algae, 6in below my MH light when it was in the tupperware bowl at the top of the tank, still dissolving.
Prior to that, I had a standard plant light on top of the sump.
Over the last year I have tried unsuccessfully at 3 different time to grow macros in my sump. They get good flow in the center of 3 chambers in a 30 gal sump, 10 hours of light. They continue to desolve away while my fish are doing well. On this last attempt I placed the alge in a tupperware...