I could propose ESAW artificial sea water [Andersen, R.A., Berges, J.A., Harrison, P.J., Watanabe, M.M. 2005 Recipes for freshwater and seawater media. In: Algal culturing techniques. (Ed by Andersen, R.A.). Elsevier Academic Press. p.494]. We have been using it for 8 years. Never had misfires. Algae from different Oceans are growing well in our lab.
We use Bacillariophycean Medium (= Diat.) and it works in most cases. The recipes could be read at http://www.uni-goettingen.de/en/list-of-media-and-recipes/186449.html.
f/2 guillard, I think is a good medium for diatom. you can use too K medium +Si which is maybe more "complete" : presence of 2 forms of nitrogen : NH4 and NO3. phosphate in glycerophosphate form