Ok then. Will add more plants to try and reduce nitrates.
Thanks Simon and JayC for the pointers and hints. I think the Dino Spit is the Glutaraldehyde and the Dino Pee has the macro and micronutrients. I will leave off the Glutaraldehyde and reduce the dose of the Pee to one third. I am pretty careful with the dosing.
Good news is that there are at least two RCS females still in the tank and looking well. Maybe they went and hid and moulded after the big water change? The surviving DAE are all looking well.
Anyway will take all your advice on board and do smaller water changes and work out a dripper system.
jayc's comments on letting remineralized water sit around got me thinking. I've had pre-mixed up batches of water sitting around for a month or more with no obvious growth. I asked the Salty Shrimp people about this, but unfortunately they replied back to me in German, and google translate doesn't seem to work too well. So if anybody knows German, here is their reply:
" prinzipiell ist es kein Problem dein mit Bee Shrimp GH+ aufgesalzenes
Osmosewasser zu lagern, jedoch werden dabei einige Spurenelemente
verloren gehen. Dies hat sich bei unds allerdings noch nicht als negativ
herausgestellt, da wir selbst meist eine Menge von ca. 1000l aufgesalzen
parat haben. "