hi
some water companies have higher nitrate levels than others which will affect different peoples readings but to lower the levels do several small water changes over a couple of days ( say 10%). how often do you do your water changes routinely???
black algae can be caused by overfeeding - too much nutrients in the tank, lighting on too long, not enough other plants to compete with the algae for nutrients available, high phosphate levels in your tank will affect algae growth ( ie: you'll get more). the fertilizer your using may be contributing to the problem. i use a carbon additive - flourish excel & an actual fertilizer very sparingly. we had a big black algae prob a couple of years ago & i got rid of it by:
1. cutting back on food ( i admit i overfed our fish)
2. adding rowaphos to our external filter (phosphate remover)
3. adding more plants
4. introducing a lighting break - our lights are on from 0800-1300 off 1300-1500 back on 1500-2000 when moonlights come on, our lights are all on timer plugs
5 i took out all rocks covered with the horrible black stuff & poured boiling water over them & scrubbed all the algae off & chucked out all algae infested plant leaves etc
i dont know what or if a combination of all these things did the trick but it worked & we dont see black algae anymore. i also have a phosphate test kit so when i think the rowaphos is due to be changed i check the levels ( i chaecked the levels before adding rowaphos the first time & keep a record of what it was when it was high &we had loads of algae)
good luck that black stuff looks terrible! we still occasionally have a small amount of green algae growing on our rocks but it looks more natural & keeps our ottos fat & happy!