I would give them 14 to 16 hours of light, remember that the energy for growth is made by the leaves from light so, all other things being adequate and up to a point, more light equals more growth.
There's looks to be a white powder on the surface of the medium in the picture which could be excess fertilizer salts (although I think you have high mineral water also). With the wide spacing in the pot and the small size of the seedlings, I would be cautious with fertilizer. You might need some if there was none already in the potting media, but it is better to err on the side of too little than too much.
