Brown algae would suggest the lighting spectrum is off or there is insufficient lighting, might be best to put the 6400k's back. The end goal is to provide optimal conditions for your plants so they can out compete the algae, you will get there and I appreciate how frustrating this must be for you.
I'm sorry if I missed it while scanning over the thread, but what sort of plants do you have in there and how many?
Potassium dosing is starting to sound like an attractive idea to me. If there's sufficient plant life and your lighting is adequate it should help. Next time you're out at Walmart or wherever, look in the salt section for a sodium free salt substitute called Nu-salt (made by the same ppl who make Sweet-n-Low so shouldn't be hard to find and will probably cost around $1-$2) it primarily contains potassium chloride. On your next 25% water change add one pinch (just what will hold between your thumb and index finger with only light pressure) to your change water, then give it a thorough stir and add your dechlorinator which will take care of the chloride ion. I'd add it again on every fifth water change, this will create a nutrient pulsing effect which the plants will cope with well, yet will prevent potassium build-up in the tank water.