Be sure you sterilize the tank water before you put them into the culture!!!!
Tank water may, and very likely do, contain rotifers, pod larvae, or other micro algae consuming fauna that will wipe out the culture when added. You should always sterilize the tank water by either passing it through UV at a very slow rate (to kill everything in the water), or put them into the microwave and let it boil for a minute. Some people use freezing method, but I never tried that myself.
I would simply skip the tank water but using the newly mixed saltwater as the source. Add chemical fertilizer as necessary. There are many companies that makes 2 part liquid fertilizer for the purpose of growing green water. Some of the garden liquid fertilizer will do the trick just fine, just use it at 1/2 the recommended concentration as a start. I do not recommend skimmate as they may, again, contain larvae that feed on phytoplankton as well.
Good luck
p.s. Becareful on the temperature, high temp will kill off the culture too. The plankton culture manual from Florida Aqua Farm is a very useful guide to show you how it is done.