I have blue-green algae in my 30L tank. It's not very serious condition now, but algae is expending it's territory on a driftwood. I went to a pharmacy to buy erythromycin but they don't cell it in useable form but another ???mycin,Doxycycline, Hyclate Cap. Is it Gram(+), or Gram(-). Can I use this? Anyone know about this?