Firstly, thanks to all for the advice to add 3 kg salt to my system to cure the sporatic fish deaths I was encountering. That has solved the problem.
My next query is regarding summer heat. I live in Perth, Western Australia and next week is supposed to be a real scorcher ... 34 on Monday, 36 on Tuesday and 39 on Wednesday to Friday inclusive!
What extra protection will my two grow-bed system require (as it is in full sun)? Should I cover everything with shade cloth? If so, the 50%, 70% or 90% variety? And if the 50%, where can I buy it?
Many thanks and best wishes for the holidays.
I didn't find the right solution from the internet.