White fungal growths

[George Farmer]

I'm getting what I can best describe as a white fungus growing on my bogwood. I have 3 pieces (of bogwood) but only 2 are affected. They are only a few small patches but slowly grow. The growths can be removed mechanically with a toothbrush but they return. My tank is well-planted, stocked with Angels, Rams, Tetras, Corys and Otos. pH 6.8 - 7.0 depending on CO2 level, KH 3.5, GH 8, Ammonia 0, Nitrite 0, Nitrate <10 ppm, Phosphate <0.25 ppm.

Any ideas please?