Oh boy... possibly identified what was causing issues in my tank. Realized I hadn't seen the other sea hare in a minute (the second one I assumed was still alive)... started looking around the tank. Finally noticed it was dead and sucked up into one of the Nero 7's.... not much left of him, had probably been there decomposing for a while, likely a few days before the other one died. Removed what was left, we'll see if things start improving after running some fresh carbon and a water change.