The five-minute test at feeding time
The clearest evidence appears while you feed. Give the usual portion and watch: a correctly sized meal is gone, actually eaten rather than just sunk, within a few minutes for most community fish. Food still floating at the surface, flakes settling on leaves and substrate, or pellets rolling uneaten along the bottom after that point means the portion exceeded the stomachs. Fish are unreliable witnesses here: healthy fish beg at the glass whenever someone passes, because begging is learned behavior, not a measure of need. Feed with a measure for a week, a small scoop or a counted number of pellets, and note the moment the tank stops eating and starts ignoring. That moment, not the packet's enthusiasm, sets the portion.
What the tank shows you afterwards
Surplus food does not disappear; it decays, and the tank displays the consequences in a recognizable order. First comes the film: algae spreading faster on glass and plant leaves between cleans, because decaying food feeds it. Then the water: a slight cloudiness the day after feeding, mulm collecting in corners and in the filter, sponges clogging sooner than they should. Pest snails are honest auditors too, since a population that suddenly booms is living on somebody's leftovers. None of these signs proves overfeeding on its own, but several together, in a tank whose stock has not changed, point one way. Waste levels creeping up between water changes tell the same story in numbers.
What the fish show you
Overfed fish carry the evidence on their bodies. Look for bellies that stay rounded all day rather than filling at feeds and easing between them, fish that are sluggish outside feeding time, and long trailing waste, since a gut working overtime produces plenty of it. Fry and juveniles should look keen and grow steadily; adults should look trim and interested, not permanently stuffed. Compare photographs a month apart if you are unsure, because slow gain is hard to see day to day. Remember that the opposite does not hold: a fish that rushes the glass is not necessarily underfed, and the slimmest fish in the tank may be outcompeted rather than under-portioned, in which case the fix is delivery, not a bigger shared ration.
Resetting the ration and keeping it reset
Reset by shrinking the portion, never by skipping days at random: cut the measured amount by a third for a week, watch the five-minute test again, and adjust until the food just runs out as interest fades. Siphon the substrate corners where old food has collected, rinse the filter media in tank water, and let the algae film tell you over the next two weeks whether the new ration is right. Then protect the fix. Pinches creep back through kindness, especially with several feeders, so the measured amount needs to be the routine, not a resolution. Recording that exact ration in KinsLe Care Teams, with each feeding marked done by the person who gave it, keeps every feeder honest to the scoop, and the cloudiness, snails and film stay away.