Hello everyone! I talked to someone at the school who knows a lot about fish, who in addition to agreeing to help with the puffer from my other thread, took a look at the foxface. He said it was hard to tell if anything was wrong with him. He also has seen him eating algae off the sand, and agrees that's why his poop has white powder. As for the brown stuff on him (just a little on his spines) he thought that it might get better once the tank is cleaned (there's a starfish that has similar whitish slime on it too). As for the lying down during the day, he suggested that it may be because his schedule is off (light timers and everything. Maybe because it's summer and the days are so long, and he's used to sleeping earlier? Just a thought). He also agreed that the temperature was a bit high, and will see what he can do about it. He said the sign was indeed out of date, but here's the data on it: 75 gallon (I think the tank itself has been changed since then. I'd say the tank is about six feet long, maybe about two feet wide, and a few inches taller than it is wide.)
Temperature: 79 F (it's actually 82)
Salinity: 35ppt
pH: 7.8-8.2
Alkalinity: 8-12 dKH
Calcium: 350-400 ppm
Magnesium: 1250-1350 ppm
The sheet also says they are fed once daily (why did I think it said twice?

), and that they clean the window everyday

as well as add major and trace elements every day.
The container with the tube is still there. I was mistaken: the tube leads into the cabinet under the tank when the filtration system is, not directly to the tank. It says it is calcium hydroxide, and appears to not be transferring any more through the tube.
As far as the tank mates, I had mentioned a goby, but I haven't seen it in at least two days, so I'm guessing it died (someone told me that at some point, its fins had seemed rather tattered because of another fish (I'm guessing the three striped damsel) although I looked at it a few weeks ago, and while it may have had a bit of damage, it didn't look too bad.)