Last nigh I startled my octo and he shot ink all over the tank. He has never done this before. The ink was all clumpy on the surface. I immediately went to the ocean and got some new seawater. I did 100% water change. Is this necessary?
My octo "Herman" did it too once or twice. I happen to have a fine mesh net and if you are quick and gentle you can actually scoop it out. I also make sure that the protein skimmer is running right and is clean. A 100% change in my case seemed to be extreme. I just stick with a 20% every two weeks. Remember that doing a 100% change is a shock to the entire bio-system because you have removed a lot of the resident organisims living in the water that everyone is used to. The ocean water has all the right ingredients but chances are the PH, salinity, and temp won't be exactly the same. Just make sure you aren't doing more harm than good. I think that if you have adequate filtration and a light bio load then you should be fine with just scooping out as much ink as possible and letting the system absorb the rest. Hell, everyone in my tank lived and that was months ago. My water is clean and the chemistry is right on. This is just opinion only. I am still fairly new at this too.
You can count on your protein skimmer to remove most of the ink. If it's the thicker, blobby type of ink, you should try to remove as much as possible with a net. We discussed the "panty hose net" a couple of years ago - a regular net lined with panty hose material, very good for ink.
If your octo squirts ink so that it's a smoke screen and the whole tank is black, you won't be able to use the net. I do a small water change - 10 or 20 percent and let the skimmer take out all the rest. Fresh carbon might help, too.
Yes, a 100 percent water change is a lot, especially all at once. However, I can't find from your past posts what filtration you're using or whether you have a skimmer.