The baseline is one specific fish
Generic recovery advice fails because fish are individuals. One species cruises the top all day while another's healthy normal is sitting motionless on the substrate for hours, and a fish that has always been a shy feeder looks identical to a sick fish if you judge it by a bold tankmate's standards. So the first monitoring tool is memory made concrete: what did this fish do at feeding time, where did it swim and rest, and how fast did its gills move, back when it was unquestionably well?
If you never noted it, build the baseline from the healthy tankmates of the same species and from how this fish behaved before it fell ill, and write it down now for next time.
The daily feed watch
Pick one feed a day to observe properly, standing still and watching the whole event rather than glancing while you do other jobs. Score four things. Does the fish notice food arriving? Does it move toward it with something like its old intent? Does it actually take and swallow food, rather than mouthing and spitting? And how much of its normal portion disappears before it loses interest?
Early in recovery, noticing and tasting is genuine progress. Across the following days you are looking for the sequence to complete: notice, approach, eat, and then keep foraging after the food is gone, which is a healthy fish's signature.
Swimming, breathing and station
Between feeds, read the body. A recovering fish holds its station in the water column more steadily, chooses its old resting spots instead of hiding in new ones, and keeps its fins open rather than clamped to its sides. Breathing should trend toward its baseline pace; count gill movements for a short timed stretch at the same time each day if you want the comparison to mean anything.
Note social position too. A fish rejoining the group's loose formation, or reclaiming a favourite cave, is telling you its confidence is returning in a way no single feed can show.
Slopes, stalls and reversals
Judge in runs of three or four days, because a recovering fish has off days and one dull morning after a water change is weather, not climate. The encouraging pattern is a slope, however shallow, in the right direction across most measures. The patterns that need action are a flat line well below baseline after the treatment should have bitten, and a reversal: a fish that was eating and then stops, or whose breathing quickens again. Either warrants contacting whoever advised the treatment, with your notes in hand.
Feed lightly throughout, even when appetite returns enthusiastically, because the recovering body and the recovering filter are both easily overloaded.
Keeping those daily scores in the fish's Pet Passport on KinsLe turns recovery into a visible line on a page, so the day it plateaus or turns, you see it the same evening rather than a week later.