I agree with the over watering theory, I grow in soil, and can tell when my plants need water, simply by the weight of the pot!, however the spots on your plants look to me as though it could be a toxic salt build up in your soil, which is stopping the roots absorbing the Nutrients in the fert, I have found that the best solution to any unidentified problem is to flush the soil, first check the PH of the run off water (The water that runs out the bottom of the pot after watering) you may be suprised to see that it is higher than the water you are pouring in the top! this can be anything upto 1-3 PH higher. This is usually a fair indication that you may need to flush.

What I do is pour 3 times the capacity of clean PH balanced 1/4 strenth solution through the pot so for a 3 gallon pot, use 9 gallons of water. this may seem a lot but use the run off to feed your lawn or outdoor plants. you should then let your plants rest for a few days ensure they are able to drain freely, then leave them until the soil is almost dry, get a feeling for the weight of the plant now, as this is the weight you should be looking for before between waterings. Remember the roots need Oxygen! if they are sat in wet soil for 100% of the time they will drown and rot! Let me know how they do.