Set up for a fish that hides for a living
A bristlenose pleco spends its days pressed underneath something, so a bare, bright quarantine tank is a stressful box for this particular fish. Give it one cave or a half coconut shell, and add a piece of driftwood without fail: bristlenose rasp wood constantly, and the fibre genuinely aids their digestion, quite apart from the security the wood pile gives them. Dim the light or leave it off for the first day. Warm, clean water in the mid-seventies Fahrenheit suits them, and because even a young pleco produces a surprising amount of waste, plan on more frequent small water changes than the fish's modest size suggests, with a filter that keeps up.
Feeding a nocturnal grazer in quarantine
Do not judge a new pleco by daytime behavior; judge it by the food that vanishes overnight. Drop in an algae wafer or a blanched slice of zucchini or cucumber shortly after lights-out, weighted so it stays on the bottom, and inspect what is left in the morning. A pleco that is settling will also leave rasp marks on the driftwood and neat piles of sawdust-like droppings, both good signs. Avoid flooding the tank with food to tempt it, since uneaten vegetables foul a small quarantine quickly; one modest portion nightly, removed by morning if untouched, tells you everything. Note that very young bristlenose need protein as well as greens, so rotate in a sinking pellet now and then.
The signs that matter on a pleco
Plecos press themselves against the glass, which hands you the best health check in the hobby: look at the belly from underneath. It should be gently rounded; a sunken, hollow belly means the fish has not been eating properly, whether from stress, competition at the shop or illness, and needs feeding up before anything else. Run your eyes over the skin and fins for white spots, pale fuzzy patches or fraying edges, and watch whether the fish rasps and moves with purpose after dark or sits utterly still in the open, which is uncharacteristic. Breathing that stays rapid in clean water, or a pleco that cannot seem to stay attached to surfaces, is a reason to test the water at once and to hold the fish in quarantine longer rather than graduating it on schedule.
Hold back the urge to treat just in case
Because a pleco shows so little during the day, keepers are tempted to treat the quarantine tank preventively to be safe. Resist it. Unneeded treatment stresses a fish that is already adjusting, complicates your reading of its true condition, and a healthy pleco needs nothing but food, wood and time. Treat only specific signs you can actually see, and if you must treat, remember that plecos are armored catfish with sensitive skin and choose the gentlest appropriate route, watching closely for any reaction. A few weeks of clean observation is worth more than a course of anything given on suspicion.
When the pleco graduates
Move the bristlenose when it has fed reliably for a good stretch, its belly is rounded, and its skin and fins are clean under a proper look in decent light. Acclimate it to the main tank's water gradually, and introduce it in the evening if you can, so it can find a cave and claim a territory while the tank sleeps. Give it its own hiding place in the main tank from day one rather than making it fight established bottom-dwellers for cover. Jot the quarantine outcome, what it ate best and how its belly filled out, into its KinsLe Pet Passport before you move it, so the fish that disappears under your main tank's driftwood still carries a known, written history.