I used Fox Farm for years too and was satisfied with the results. Talking hydro here. I was satisfied because I didn't know any different, and it was hugely different to the soil grows I was used too. It left me with deficiencies, specifically iron and nitrogen. I am in no way an expert at deficiencies, but when I tried supplements the problems got better and I became more than satisfied. After much reading and asking around I'm about ready for a full on Dutch Master grow, with a little Advanced Nutrients thrown in at the end. Those are two names that will not steer you wrong IMO and in that of many others more knowledgeable than I.

I won't try and tell you what's causing yellow spots because I've decided to not give advice if I'm not sure about something. I don't want to risk someone else's grow. The best advice I know to give is to research via the www. I'd start with this forum and search problem chart, since there are plenty. I know I've posted at least 5 in the problems section.

How about some pictures of the problem?