The thaw water will still have some of the pack juices that the frozen foods were made with. These pack juices are said to be full of phosphates witch is one additive you should be trying to minimize introducing to your system. (micro algae’s love it!)
When feeding thawed frozen foods, I change out the water that the foods are thawing in with a fresh batch of water a few times. Usually the frozen food will sink to the bottom of what ever container you’re using, and you can pour off the top water and add fresh water to dilute the amount of pack juices. When I feed, I use a small syringe with a piece of ridged airline tubing attached to it so I can target feed and have better control on the amount that I’m feeding.
hth