ToxicTBag - been running test this morning on the fluids.
Flux ... Ive filtered it down and stuck it under the microscope and we can see tiny particles of sticky flux so it looks like even though you've done your best to get what you can out, you may still have this problem for a while as the flux is still leaching into the water. The best advise for now is run your system with water + biocide for 1 month or even premix (if you need some just gis a shout ill bring some up) then flush again then switch to pastel. The partial count is also though the roof as is the PH of the liquid which explains it all.
If you need any thing else don't hesitate to contact me.
Btw every one what a nice bloke
and make brill cups of tea...