It's nothing so don't go throwing shit at it to fix what isn't broken or you will break it.
Look at the healthy newer growth. Snip off the bits that are damaged so you can tell if it gets worse. They're older leaves and don't mean shit.
If anything add a tsp of epsom salts to 4L of water next watering. Just every 3 waterings like you're doing it now but one where you are just using water.
They look fine and are of a size that they should really get going so you may have to feed a little more or do the feed - water - feed - water thing.