Over or under watering

Soil? defintiely overwatering. Big pot, small plant has additional concerns. You can do it, but it's more guesswork. If you are fertilizing often, the plant doesn't have a root system to cover that much soil or anything for that matter. So, it just gets unused on top of evaporation and concentrations rise quickly. So, water near plant -- enough so that you have at least 24 hours before next watering, preferably you only have to do it every other day in soil. If the outside of pot starts to get dry, hit it with some plain pH'd water to rehydrate it. As the plant grows, water/fertilize as normal a larger radius around the plant. If that's coco, you can water more often, but still want top layer to dry a bit, so you don't grow algea on your perlite - and anything else far worse. as far as 18" depends on light.. if you see nodes/leaves stacking on top of each other with no space, move to ~24". otherwise, it's fine.