My first Grow

6.5pH is too high, even for "regular soil" I wouldn't go any higher than 6.2. For your choice of grow medium, I'd personally aim for 5.9/6.0 every time. Also, give your plants some sleep! With an 18 on 6 off light schedule. I don't have tons of growing experience but I have proven with my clones (at least the strain I'm growing) that roots definitely grow faster with a sleep period. I've also noticed that giving them a sleep period helps them recover from transplants, toppings, etc faster. I'm trying to grow my clones slowly on purpose, so I'm actually purposely giving them 24 hours of light after having experimented. The exception to this is after I top them or remove any leaves etc, I put them in a dark cabinet for a few hours.