LK is organic to a mystical degree and it does leave a nice brown like crud on your roots, but it has happen to everybody's roots that I have met or read about that use LK. As for your pump clogging, are you using a bubbler with stones, which would only affect the air stones, or are you using a water pump? How big of res are you using? LW uses bubblers with this formula and not a water pump. I imagine that if you put an air stone in the res it would allow the LK to stay more liquefied and not settle into the organic slush, clogging up your pump.
Hope this helps.
Adieu