What the skin is telling you
A green tree frog breathes and drinks through its skin, which makes that skin the first place illness shows. The underside and inner thighs should look pale and clean; reddening there — the pattern keepers call red-leg — signals a bacterial problem that can move fast in a small frog. Dull, greyish or patchy skin, skin that sheds in ragged excess instead of being neatly eaten, and any sore or fuzzy spot all deserve attention the day you see them. A fungal skin disease that circulates among amphibians can also present as lethargy with skin that looks dusty and fails to shed cleanly. None of these can be sorted out by eye alone at home, and that is the point: skin change in a frog is itself the symptom that earns a vet call, because frogs rarely show you anything earlier.
Bloating and the water behind it
A tree frog that looks puffed up, with legs and body swollen as if filled with fluid, is showing edema — a sign linked to bacterial infection, failing organs, or water that has become fouled enough to poison the animal sitting in it. Amphibian skin absorbs what surrounds it, so untreated tap water, soap residue, aerosol sprays used near the enclosure and decaying food left in the water bowl all become health problems, not just housekeeping lapses. A frog in toxic trouble may sit stretched out stiffly instead of tucked into its usual sleeping posture. Move a bloated or stiff frog into clean, treated water in a bare temporary container and contact an exotics vet the same day; the water change is first aid, not a cure, and the underlying cause still needs finding.
Bones and feeding in a very small frog
Calcium shortage creeps up on tree frogs fed the same few feeder insects without dusting. The signs are a frog whose grip weakens until it slides on leaves it used to hold, strikes at prey that fall short or miss, and in advanced cases a jaw that looks soft or misshapen. Dusting feeders with a proper calcium supplement on a routine and offering variety are the prevention, along with the low-level ultraviolet light these frogs use even though they are nocturnal. Mouth injuries from lunging at hard enclosure fittings, and a frog that gradually stops hunting while tankmates thrive, also belong on the watch list. Weighing a frog this small is impractical, so appetite and grip strength are your gauges — which is why noting them matters.
When a frog needs a vet quickly
Frogs decline in days where a lizard might decline in weeks, so the thresholds sit closer: a green tree frog that stops eating for more than a couple of days, sits on the enclosure floor instead of climbing to sleep, shows spreading skin sores, or looks bloated should be seen promptly, and a stiff or unresponsive frog is a same-day emergency. Bring your notes when you go — water source and change routine, temperatures, what the frog last ate and when the skin changed — because with frogs the history often points at the cause faster than any test. Keeping that running log of water changes, feedings and skin checks in your green tree frog's KinsLe Pet Passport means the evidence is already written down on the day you need it.