[Delphinus]

Nori maybe? But it's still seaweed. There aren't too many susbstitutes (if any, really). You can try feeding romaine but there's debate as to whether that's any really any good (nori is better, at any rate), and I doubt all herbivores will really take to it (tangs maybe). I used to feed romaine but I don't anymore, my tangs are WAY fatter on nori.

I dunno ... feeding seaweed shouldn't really spike your system. How big is your tank? How are you feeding (and what seaweeds in particular)?

If you're getting a spike I wonder if there's a lot of it that's going uneaten. I feed my herbivores nori rolled around a piece of PVC. When I used to just feed a loose sheet held in place by a feeding clip, they would make such a mess of it. Little bits everywhere. Of course they can't eat it all, some of it would disintegrate. Since switching to the rolled up method, I don't really notice little bits floating around anymore. All that comes off the tube, is the stuff they eat. Thus, they "harvest" more of it, less of it lingers and potentially fouls. Thus, better water quality and fatter fish. Try it this way, if you aren't already.