byte       led[] = {3,5,6,9,10,11};
int brightness[] = {0,0,0,0,0,0};
int  increment[] = {1,1,1,1,1,1};
byte    lenght[] = {1,1,1,1,1,1};
int       walk[] = {0,0,0,0,0,0};

void setup()  { 
  for (byte i=0 ;i<sizeof(led) ;i++) {
    pinMode(led[i],OUTPUT);
  }
} 

void loop()  { 
  for (byte i = 0; i < sizeof(led); i++) {
    analogWrite(led[i], brightness[i] > 0 ? brightness[i] : 0);
    if (walk[i] >= lenght[i]) {
      walk[i] = 0;
      brightness[i] = brightness[i] + increment[i];
      if(brightness[i] <= -256) { 
        brightness[i] = -256;
        increment[i] = -increment[i];
        lenght[i]=random(0,15);
      }
      if(brightness[i] >= 255) {
        brightness[i] = 255;
        increment[i] = -increment[i];
      }
    }
    walk[i]++;
  }
  delay(1);
}