That's all very interesting stuff John.  I obviously don't need it but I can attest to the problems caused by algae.  I pump up from my well into two cubo's in the garage connected in parallel.  The output is pumped to the taps/showers in the house.  The pipes from the pump to the taps run for about five meters along the inside of the garage wall and are the clear plastic type.  After a while, blue/green algae (I think) built up in the pipes to the extent of completely blocking one of the taps.  Happily I was able to dislodge it by moving the flexi connector about.  I flushed the system through with bleach which seems to have removed most of it and will lag the pipes later to keep the light out.
You mention sharp sand.  Do you know what the local builders merchants call it please ?