Root rot or something else?

Yesturday i noticed my lettuce leaves looking a bit sad but roots were white and nice, some tiny brown spots on a few but i didnt even think about it.

Today half of the plants had brown roots and leaves with yellowing tips, the recirc waterpump had brown sludge in the filter. Did a deep clean, replaced nutes and did individual h2o2 treatments and now tiny amount added to new nutes. What is this and where did i go wrong? Worth mentioning this is a brand new system, not even 2 weeks of use