Thanks everyone for your most valuable input.
I'm thinking Red may have the answer here, the CO2 levels always fluctuated,
and the low light plants grew in fits and starts.
Since disconnecting the CO2 and adding the zeospeed to the canister, my
phosphate levels have dropped significantly. I'm using the Hagen Nutrifin
test kit and this morning it's extremely difficult to detect any colour in
the sample, it just appears slightly milky.
The black brush and beard algae are turning very pale and I'm now wondering
if this is possibly a sign of their imminent demise?
Assuming it is, does anyone know if it will slowly decay and dissapear or is
it likely to remain, kind of like a dried arrangement?

Thanks!