Yet another super dark greenwater haverst!

I have  a 75g, how would I feed this ?

 

turn all your pumps off and skimmer and pour it in ....

 

How much for a 75g?

 

I would only turn off your return, if skimmer is sump fed leave it on as it's only processing sump water. Keep circulation pumps on maybe in feed mode (if applicable) to start then allow to run normally as long as you feel comfortable. Keeping circulation pumps on allow corals to feed and clean themselves. It will also allow other filter feeders like sponges, tunicates, zooplankton to consume it.
I would recommend starting with something like a cap or two and see how things respond to it. Phyto can stimulate feeding in corals that would not consume phyto alone so you can take advantage of that by adding other coral foods you might use. Be careful when feeding a tank heavily with anything as it can cause skimmers to lose a head or foam rapidly depending on protein content, oils, etc.