I also add dry ingredients sometimes, but I just guesstimate. Like I said, they can tolerate a wide range as long as they're not drowning.
I haven't considered seaweed before, but I think they would eat it. The only reason they wouldn't is if it was too high in cellulose. It might just depend on what type of seaweed. I wouldn't worry about rinsing it well, but soaking would make it easier for them to eat.
