I don't add any Epsom but have added lobster compost and EWC at topdressing.. I also went back to using tap water but i cut it by 75% <100ppm...
I hate to burst your bubble and maybe I am wrong and someone will correct me. But the Seakelp although it has a NPK rating the amounts are minuscule.. I don't believe it will work as a P-K booster like you hope.. Kelp is a great growth hormone don't get me wrong but I have not heard of anyone using it as part of the nutrient regime