Plant for a fish that works for a living
The siamese algae eater is bought as an employee: a tireless grazer that rasps algae, including the stubborn black beard algae most other fish ignore, from leaves, wood and glass. Its planting therefore has a job specification. The leaves it grazes must be tough enough to survive constant rasping, broad enough to hold film for it to find, and — once the fish reaches adult size — strong enough to rest on, because a grown algae eater drapes itself across a leaf like a hammock while it works. Fragile planting fails this fish twice over: there is too little grazing surface, and what surface exists gets shredded.
The grazing platforms that hold up
Anubias is the outstanding choice, its leathery leaves thick enough to be grazed daily for years without holing, and its slow growth means the film it carries is rarely outpaced. Sword plants (Echinodorus) serve the same purpose at larger scale, each long leaf a grazing strip at a different height. Java fern's tougher foliage also stands up well. Behind these foreground platforms, Vallisneria and fast stem plants such as water wisteria do the background work — they grow quickly enough that incidental grazing never thins them, and their roots and leaves trap the gentle current these river fish enjoy.
Layout for a long, active swimmer
Siamese algae eaters are streamlined river fish that grow larger than most buyers expect and patrol the whole tank at speed. Plant the back wall and both ends densely, then hold the front and centre open as a swimming lane, or the fish will be forever barging through your foreground planting. A piece of driftwood running lengthwise gives them a resting and grazing station mid-tank — they are often found lined up along a log, resting between grazing bouts. In a shorter tank a single algae eater is plenty; these are social fish, but cramped quarters turn their energy into restlessness. Length also protects the planting itself — a fish that can cruise a long lane spends its energy swimming, while one boxed into a short tank spends it barging through stems.
The older-fish caveat, honestly stated
A young siamese algae eater is an algae specialist, but as the fish matures it develops a taste for easier food — it will queue for wafers and frozen food, graze less diligently, and may begin rasping at soft plants and even the slime coat of slow tankmates if genuinely hungry. Delicate mosses suffer first, often eaten outright by a large adult. Keep the adult fish well fed on vegetable-rich foods, keep its planting on the robust side from the start, and it stays the useful, peaceful grazer you bought. Do not starve it into working; a starved algae eater damages the tank it was hired to clean.
Grazing surface, toughness and growth rate pull in different directions for this fish. When a particular plant is in question, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.