I asked my father if he could remember more about the Tauranga bore water. The brown sludge I was remembering was organic iron, and he thought the silica came from sand from higher up the bore - and therefore likely to be present for anyone with a shallow bore. He doesn't remember it having any effect on algae, except that we did have an algae problem with what sounds like cyanobacteria or similar. He couldn't remember exactly what it was.
Re what I was saying about plants v. algae competition, the advice I remember from the book was basically that any plants that either have roots growing free in the water, and/or are at the surface or even above it are best. Also that the most important thing is that the plants are growing actively. One of my tanks that was very green for quite a while has now been good for several months and all the plants are growing beautifully. I use the prunings in the other tanks, two of which have also now cleared of green water and the plants grow well. The fourth (and newest) tank struggles to grow plants and is pea soup green within a couple of days of a water change. A partial change makes no difference at all - just feeds the algae more I think - and a complete change of water starts greening again straight away.