Jukenjer, I'm south of you, just north of Mille Lacs. First batch on the year for me was dirty. It was only 100 gallons which once the first pre-filter plugged I pulled it out until the second plugs and so forth. One year i recall using six pre-filters, this year only four needed so far.
You tried to filter four times that amount i did. I can understand your frustrations unfortunately more pre-filters needed. Once there plugged cant force more thru.
Its nothing your doing, i call it a dirty batch. Happens once and a while for me.
If someone has a better explanation to why the dirty batches happen I'd be glad to listen.
Hope that helps
2008 - was the start of it all!!!
2016 - New 22" x 34" flat pan
2018 - 50 bags and new RO system.
Still a great hobby.