First, make sure you bought the real fish
The name siamese algae eater is attached in shops to several different fish, and only one of them is the tireless algae grazer you want. The true species carries a black stripe that starts at the nose and runs the full length of the body, continuing unbroken into the lower half of the forked tail. Its fins are clear, without yellow or black tinting, and when it rests it props itself on its fins at a slight angle rather than sucking onto glass. Common impostors include the false algae eater, which develops a sucker mouth and a taste for other fish's slime coats as it grows, and various flying foxes with tinted fins and a stripe that stops at the tail base. Check your fish in the quarantine tank under good light before anything else, because quarantining the wrong fish perfectly still leaves you with the wrong fish.
A quarantine tank for an active grazer
This is a busy, oxygen-loving fish from flowing water, so set up quarantine with a snug lid, since a startled algae eater can and will jump, and a filter that keeps the water moving and well aerated. Give it surfaces with a job: a stone or piece of wood carrying some soft algae is ideal, and if your quarantine is too clean to grow any, offer blanched zucchini or a portion of algae wafer so the fish can demonstrate the grazing behavior you bought it for. Warm water in the mid-to-upper seventies Fahrenheit and a few weeks of this routine also let a young fish put on condition, since shop algae eaters are often skinny from competing in bare sale tanks.
What healthy looks like in this species
A healthy siamese algae eater is almost never still for long: it cruises, perches at its odd angle, and works surfaces with constant nibbling. The stripe should be bold and the body filled out rather than pinched behind the head. Warning signs are the universal ones, white grains on the fins, scratching against decor, labored breathing, but add two species-specific checks. First, appetite for algae and vegetables should be obvious within days; a fish that ignores all food is a fish to watch. Second, look at the mouth: a true algae eater keeps a small mouth with a single pair of tiny barbels and never develops the downward sucker disc of the false species, which becomes clearer as impostors grow.
Into the main tank: algae patrol begins
Graduate the fish when its identity is certain, its body has filled out and it has grazed actively for a good stretch with no signs of trouble, then acclimate it slowly and release it with the lights low. Set expectations honestly from the start: young siamese algae eaters are famous for working through hair and brush algae, but as they mature they grow lazier and develop a real fondness for fish food, so keep feeding the tank's meals away from it where you can and continue offering vegetables. In the main tank it needs swimming length and a lid just as much as it did in quarantine. When doubt remains over the identification check, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library, and the fish stays in quarantine meanwhile, where a mistake costs nothing.