Could be high phosphates but I do regular WC so I thought that would remove them.
This is an unplanted low tech tank, and from numerous web pages I have now read, it is said that algae flourish when there is high lighting and low nutrients. To combat the above I need to cut back on the lighting and feed the few plants I have. So, based on the advice also given by others above, I have started to add some Flouish Excel. Took me 30 mins just to get the cap off though.