Frequency really depends on the quality of the soil. I don't use nutes anymore, because I blend my own soil. It has everything the plant needs, and they grow crazy big. Checkout subcool's recipe.
Once a week is good if youre using a good organic. You could bump that up to every other watering, or even every watering if you don't use full strength nutes. It really depends on the plants and what's already available in the soil. Start at 1/4 strength once a week,.. then after the plant is over this issue with the spotting and a little more mature, then you can start to provide more nutes. They'll thrive when they're happy, and let you know when they're not. Don't try to push them to the limit and you'll have a good grow. People run into issues, when they try to make them grow as fast and big as possible. Instead of increasing NPK, you may want to think about trace elements, and other things you can get from using Molasses as well. A little worm poop will also add good bacteria,.. If you can optimize your plants nute uptake, with bacteria to make things useable for the plant and providing other elements that they need to process NPK, you'll find that you don't need as high an NPK in your waterings.
Adjust the water pH to 6.5.
Water alone is not pH buffered very well unless you're using well water or hard water high in alkalinity. Soil can be very well buffered. For example, It may take a lot of pH 6.5 water to change soil that is 7.2 depending on the amount of carbonates, hydroxides and such in your soil. If you start at the right pH you should be fine. Check the runoff also.
