Crees que es carencia de potasio ??

I see you are in soil, if this is the same plant as in your diary. 6.2pH might be a bit low for soil. I'd raise that up to 6.5 range and see if problems fix themselves. Then observe and adjust from there. lack of potassium usually causes older leaves to turn pale, first. Fast and new growth can sometimes be a little yellow to start but if it fills in quickly, that's fairly normal. Some tips are pointing downward, and that's a sign of too much, not too little. But, i'd get your pH into a better "happy" zone, first. Then see what happens... of course in the meantime you must keep an eye on it... at the least, it doesn't look to be spreading quickly, so you can be conservative about it.