Thanks for your input. @taricha, believes it actually might be prorocentrum, but is telling me diatoms could still be useful allies. I see you are based in the UK - may I ask how you sourced Sponge Excel? I can't seem to find it over here?
Brightwell Aquatics SpongExcel 125ml
Ionic Silica supplement for Increasing Growth Rates of Sponges & Gastropods
krakencorals.co.uk