If you've got 3 weeks, and your filter is functioning reasonably well, you should be fine.

Grey is dead; green is live. (tho, I think you'd worked that one out ;-)

RAISE THE PH!

Adding chlorine to ammonia in a HIGH ph solution produces monochloramine, which is GOOD at penetrating clinging algae. Free chlorine is not. Monochlor is what Yellow OUT leads to, IF you follow the directions. It does work; it just leaves a mess to clean up. But you've ALREADY got the mess, so no worries there.

But, if you don't raise the pH, you'll tend to produce di- and tri- chloramines which are stinkier and are NOT good for anything that I'm aware of.

20 ppm is about right. If you brush and stuff, it will drop very quickly. But, you should probably stay out of the "brown" OTO range. I think 20 is dark dark orange.

Ben