millis() at FastLED

Prompt as in the following code delay replace the millis

for(int i = 0; i < 20; i++) {
   leds[i] = CHSV(0, 0, 0);
   leds[40 - i] = CHSV(0, 0, 0);
   LEDS.show();
   delay(30);
}

I, Septillion, hereby grant you the right to do so.

And if you want help, read how to use the forum. That will for example tell you to post all the code, Snippets go to http://snippets-r-us.com. And to post code in code tags and to show us what you tried, what you expected and what really happened (does not work NEVER is the answer).