The worker that starves in a clean tank
The most common siamese algae eater problem is an empty stomach in a spotless aquarium. The fish is bought young to eat algae, does the job well, and then — in a mature, clean tank — runs out of its main food while the owner assumes it is still self-sufficient. Over months it thins along the back, grazes half-heartedly and loses the constant motion that defines a healthy fish. Unlike most illnesses this one has an owner's fingerprint on it, and it reverses simply: wafers, blanched vegetables and sinking foods offered deliberately, not leftovers by luck.
Behaviour changes that flag real illness
A healthy siamese algae eater is in near-constant motion, grazing surfaces and resting briefly on broad leaves with fins relaxed. A fish that parks in one spot for hours, clamps its fins, or rubs itself against wood and stone is uncomfortable in its skin — flashing like that often precedes white spot or another skin irritation by a day or two. White grains across the dark horizontal stripe and fins confirm white spot, most often within a week of arrival or a cold, oversized water change. Loss of appetite in a fish famous for eating is itself a headline symptom.
Social stress in a fish sold as peaceful
Young siamese algae eaters tolerate company, but adults kept crowded — or mixed with similar-looking species they mistake for rivals — can chase, nip and wear each other down, and the loser shows it in torn fins and hiding. True siamese algae eaters are also frequently confused with more aggressive lookalikes at the shop, so a fish that terrorises the tank may simply be the wrong species wearing the same stripe. Either way, chronic stress suppresses the immune system, and the 'disease' that finally shows is often just the stress arriving at its conclusion.
Ageing is not illness — learn the difference
A siamese algae eater that has grown large over years commonly slows down, rests more, prefers soft foods to scraping algae, and develops a heavier body. That gradual retirement is normal. Illness moves faster and adds signs: weight loss over weeks rather than years, flashing, clamped fins, spots, or a fish that stops coming to food it used to race for. Comparing this month's behaviour with last month's, rather than with the juvenile it once was, keeps you from treating age — or missing disease.
When the vet should see this fish
White spot spreading despite stable warm water, open wounds from fighting, a fish that has stopped eating for several days, or rapid breathing with the fish propped at odd angles all warrant an aquatic vet's input. Bring the tank's recent history: new arrivals, temperature swings, changes in the algae level and what the fish is actually fed. Your KinsLe Pet Passport notes on appetite, activity and water tests give that history dates and numbers, which is what separates a quick answer from a shrug.