For seed starting, I always open a fresh bag of potting soil, just to be on the safe side. My materials are strictly organic but I don't have a damping off problem, either. I must admit though, I had a major foliage disease problem one year when I did a top dressing with some bagged compost that had been laying around the shop yard - some had actual hay growing inside the bag - and I don't know what nasty stuff... when they say 'special'... I now beware.
I think you're having trouble because of the glum weather - it amazes me how little water tomato seedlings need when it's cold, not to mention cold and damp too. I never water them unless the soil is dry to the touch. I don't give my tomato seedlings any liquid ferts, I just use a bit of bone meal when I pot them up and that is enough to get them to the transplant stage.
BTW, when I use fish emulsion I often use a little molasses with that, to supply some potassium.. and as a bonus, it also takes away the smell.
