PO4 and NO3 should be fine if they remain there when the “brown goo” is gone since they are likely feeding on these nutrients. I would vacuum what I could during water changes and see if you can get an evaluation using a microscope.My PO4 was .0-.25 and no3 was 10 last night
I feed food cubes every other day currently
I would get an ICP also, the ATI Lab Kit will test your RODI (distilled in this case) also. Sending a sample of the distilled brand you normally use may shed some light on what could be causing the issue.
I used distilled water for years and looking back I was always dealing with something, getting a proper RODI saved me a lot of headache.