Hi, for a speedy service try www.freedomseeds.com they have a good selection of breeders, not as many as some other seed banks but worth a look. You also have Gorilla (http://gorilla-cannabis-seeds.co.uk/), they are good but take a while to post out. Herbies are good...
I'd say both over watering and nute burn, leave it to dry out properly, give it ph'd water for few weeks, your soil should have enough nutes in to last u about a month
Some people take all the fan leaves off to let more light through, as long as u have sugar leaves it can still use the light and put a bit of weight on