I'm trying to isolate actinomycetes from marine sponge samples and the media I'm using is starch casein agar with nalidixic acid and cycloheximide to inhibit the growth of Gram-negative bacteria and fungi, respectively. I want to know which is the minimum concentration of nalidixic acid and cycloheximide that I should add to the media, since I'm working with environmental samples. The last plates I inoculated (more than a week ago) have not presented any growth and I'm worried that the initial concentration I used is too high. If there's a table with standard concentrations for antibiotics, I would greatly appreciate it if someone could let me know.