I don´t have experience in keep these anemones.......
By I have a Theory........
You are in face of a high M.O. load............leading to a bacterial boom.......and ending with generalized infection..............
My theory is that you have to increase aerobic filtration..............using Trickel Filter............Dry-Wet..........
I found an intresting self cleaning model..........used in fresh water heavy stocked tanks........
https://www.youtube.com/watch?v=xbWQeZ6AzvM
This could be used with siporax.............to increase the area of aerobic filtration.........but a good quality Bio Ring works well...........
The skimmer I think that it is best used before the Bioreactor...........once the skimmer remove M.O. before it be metabolized in the Bioreactor..............
This could be suplemented with an ATS.........to deal with Nitrate excess and Phosphates.........
Also I think that the sand bed contributes to harmful bacteria grow in anaerobic conditions..............specially in a tank full of Anemones in healing process............
DSB is OK for most of systems..........but the system designed to Mags........I prefer Bare Bottom......................and a large plataform used as a hard scape.........to a perfect anemone settling.............
I think that Bare Bottom tank..........with a agressive filtration............is the best choice to deal with this kind of Organic wastes...........that came from anemone mucus.......or healing wounds from the split............
As well as the heavy feeding............
I do not trust in only Skimmer.......Live Rocks......and DSB.............
I think in someting little more agressive............like Dry Wet + ATS...........
You can use a remote DSB...........this avoid detritus traping as occours in Display DSB...............
Its just my theory..............about the link between filtration and infecctions...........
Passive filtration = infecction...........Agressive filtration = health..........
With no anaerobic spots in the tank...........
OBS : The light system have to respect the Saturation point at 400 micromols........
350 is a nice PAR reading................this is about de Zooxanthellae colonies..........
In Haddon´s tank.........or LTAs tank......you have to keep a DSB...............but deal with Mags I think in different approach...............
Its just theory.........