I tried all these things, use to cut up the nori etc. Screw all that, let me tell you how to really do it and it not be a pain in the butt.
Materials needed:
1 6" PVC pipe about 3/4" in diameter.
Some reuseable zipties - you can get some on
Amazon really cheap and have like 100 of them, they are useful for so many things.
2 coated magnets at least 3/4" diameter.
super glue.
Optional but recommended: PVC Cap.
Step 1: Screw/Slide the PVC cap on the PVC pipe.
Step 2: Apply super glue on the non capped end of the PVC pipe, put it on top of one of the magnets standing up. Let it dry.
Step 3: Take a piece of nori, fold it in half. I use half a sheet, but you can use a full sheet. Wrap it around the pipe nice and tight, 2 or 3 rounds.
Step 4: Take your zipties(I use 2) and wrap them around the nori, tighten up as much as you can. Cut off the excess ziptie, but leave an inch or 2 extra so you can reuse it. Fish have no trouble with them sticking out.
Step 5: Put it inside your tank, and use the other magnet to hold it up. Use the outside magnet to move it around and put it in place. I like to put mine on my back wall because it has a tiny profile that way, only the 3/4" diameter, and I can also hide it completely behind rocks.
And that's it. Easy to take back out when you want using the magnet on the back, no need to put your hands on the tank. The nori is held tight against the pipe, so the tangs only get their bite and not a lot of extra. It's basically like when they eat algae off a rock. I have 4 tangs, they all feed off the same pipe and they all get plenty. 0 waste.
It only takes a minute or 2 to replace it. The only warning I have is to keep your magnets from touching. Sometimes it's ok, but sometimes I've had to reglue after due to tryign to seperate the magnets. It didn't take me long to get the hang of keeping them apart.
I'll try to remember to take a picture tomorrow.