Hey Mate it can be trial and error with outdoor Hydro growing. I usually change nutes every 2 weeks in my recycle system(using tap water).
Hey you mentioned Black spots so I am assuming you are getting Black Spot fungal disease. In my experience this is usually caused by the plants being too wet which can also cause leaf drop.
Maybe try cutting back on the amount of watering they get seeing it is not so hot now. Also you can put through something like Growth Technologies Oxy Plus which is a Sanitiser(50% Hydrogen Peroxide).
I use this to keep out any Fungal issues but just remember it will kill good bacteria so only use when there are Fungal issues or for cleaning out the system.
Also good aeration in the reservoir is essential and the plants love it. I have used a Dutch system called Bato Pots in the past and did not like them. It had some of the best growth I have ever seen but they have a reservoir at the bottom of the pot and the Chilli got wet feet and had big issues with Black Spot Fungus.
Once you use Confidor you will never go back. I spent many years battling them only to watch the life get sucked out of my plants, these days I hardly even see one
Good Luck and hope this helps