Cowfish!

The littlest (what? The spellcheck isn't complaining? "littlest" is actually a word?!) cowfish (read: Panda Cory) is not doing so hot. We found him laying on his back yesterday, but a tap on the glass scared him back into a right-side-up orientation. A few minutes later, we found him sucked into the filter intake, but after turning the filter off he swam away.
Worried that he was infectiously sick, we quarantined him, and that's when we noticed (1) the water has a distinct blue-green tint and has an ammonia concentration of 0.25 ppm and (2) this fish's spine is a bit bent (though he seems to be able to straighten it) and he seems to be missing a pectoral fin.
So we did a 30% water change (the spare-tank setup makes this easy; dump the new water in the spare tank and it gets cycled into the populated tank over several hours) and hoped for the best.

This morning the cowfish still seems okay. I put in a small shrimp pellet, but I think that was a bad idea because I came back to see him doing barrel rolls in the quarantine bowl. The bowl (a yogurt container floating in the main tank) is pretty small (860 mL or 0.23 gallons). So I netted the fish out and replaced the quarantine water with main tank water. He looks okay now. I don't know how we're going to feed him, though. Maybe the algae wafers will be better. Or we could give him flake food, because there's nobody to compete with him.
He is showing some fraying at the ends of his fins. Fin rot? Or physical damage? If it's physical damage, is secondary infection an issue? So we decided to dose him with antibiotics (E.M. erythromycin).

Here's the plan: the package calls for 1 packet / 10 gallons (at 200 mg/packet, this makes a 5.3 ppm solution), once a day for 4 days. We're going to make a concentrated solution (21.2 ppm) and mix that into the quarantine water. What should be our target concentration? What concentration vs. time profile does the recommended dosage produce?
Wikipedia says erythromycin is metabolized through methylation in the liver and eliminated through bile, with a  half-life of 1.5 hours. But we have a very small fish in a lot of water, so is metabolism significant? Are there other things that may metabolize erythromycin? What about elimination? The instructions are very clear to remove any activated carbon filters during treatment. So without any metabolism or elimination, and with the specified water changes:
Day    ppm
  1    5.3
  2   10.6
  2*   8.0 (after 25% water change)
  3   13.3
  4   18.6
  4*  14.0 (after 25% water change)
This article gives 370 mg/kg as the LD50 for erythromycin injected into rainbow trout at 13°C. That's a pretty high upper limit. So even if "no metabolism/elimination" isn't an accurate assumption, it may still be a safe one...

update: SC pointed out that it actually wouldn't be too hard to split up the powder in a packet into 4 equal sized doses, so we'll just do that. We'll dose the water in a bucket at twice the specified concentration (so, 1/4 packet an a time into 1.25 gallons) and then dilute that by 1/2 when mixing into the quarantine bowl.