blue green algae and black hair algae problem?

Hi, can anyone tell me why I've got problems with blue green slime algae and
black hair algae?. the blue green algae's got so bad that after only three
days it has recovered eight square inch's of the gravel in my tank and has
grown back in every place and more on my plants!. I no that I have not been
over feeding or over stocked the tank with fish. the nitrite and ammonia
levels are at zero. the nitrate never gets higher than 5 mg/l and the
phosphate is around 0.25 mg/l. I guess that the
ph number is high and also the gh level because of the local water coming
from reservoirs on chalk/lime hills near my home, but I have not tested for
those. the tank is out of direct
sunlight and has the standard juwel lighting and reflectors that come with
there tanks. I've been thinking about replacing a few of the plants with the
fastest growing ones that I can find, but I think that unless there is a
treatment for sale in the UK "were antibiotics are not available" that will
kill it off completely it will just
start to cover the new plants, and I will be forced to remove all of them.

oh, and I change 25% of the water in my tank once a week, and I also clean
the gravel once a week with a power gravel cleaner. and my tapwater reads
zero for nitrite ammonia and nitrate and 0.25 mg/l of phosphate. thank you