Maybe that big 80% weekly water change takes too many nitrates out of the water, as ceg4048 mentioned low nitrates as a possibility -- Dusko's article also said low nitrates is a common cause. Seems BGA has the ability to grab N out of the air somehow, so it can do well in a low nitrate environment. You would think that your plants would be suffering or growing very slowly in a low nitrate environment, however.

Be sure to calibrate your nitrate testing kit -- I did this recently. It was easy to do.