First, be sure you have the real fish
Compatibility advice for this species starts with identification, because shops mix three similar fish. A true siamese algae eater has a black stripe that runs unbroken through the tail fin, a slightly bristly snout, and clear fins. The flying fox, often sold under the same name, has coloured fins and a stripe that stops at the tail, and it grows up territorial and quarrelsome. The Chinese algae eater is a different fish again and becomes a genuine menace as an adult. A true siamese algae eater is a peaceful, social grazer that does best in a group of its own kind in a roomy tank, growing to roughly six inches, so its companions should be chosen for the adult fish, not the two-inch juvenile in the bag.
Tank mates that suit a siamese algae eater
This fish lives life on the move, grazing surfaces, resting on broad leaves, and dashing about in short bursts, and it suits tank mates with the same energy. Zebra danios, rainbowfish, and other quick mid-water fish are completely unmoved by its activity. Peaceful barbs such as cherry and rosy barbs, most tetras and rasboras, and livebearers all coexist without friction. Corydoras and loaches share the lower levels amicably, since the algae eater grazes surfaces rather than defending the floor. It will live happily with snails and shrimp, and a group of siamese algae eaters together is more settled and entertaining than a single fish patrolling alone.
Tank mates to think twice about
The documented problem pairing is with slow, flat-bodied fish. An older siamese algae eater, especially one short of algae and green food, may develop the habit of attaching to the flanks of angelfish or discus to feed on their slime coat, which stresses the victim and opens the door to infection. Long-finned, slow swimmers such as fancy guppies and bettas can also be pestered by a restless adult. Aggressive territorial fish are the reverse problem: a pushy cichlid will corner a grazer that has no interest in fighting. If you keep flat-bodied fish, the honest answer is to meet your algae needs with otocinclus or snails instead.
Room, food, and behaviour as it grows
Many problems blamed on temperament are really hunger and crowding. A siamese algae eater that has cleared the soft algae will go looking for food, so feed it: algae wafers, blanched vegetables, and a share of what the other fish get. It is also a determined jumper, so a tight-fitting lid is part of keeping it with any tank mates at all. In a long tank with flow, plants, and enough to eat, its grazing is spread thinly across every surface rather than concentrated on a tank mate's flank.
Tracking the fish as it changes
Because this species' behaviour shifts as it grows from a diligent juvenile into a large adult, the pairing that works this year deserves a second look next year. Recording its arrival size, growth, and any flank-nipping incidents in its KinsLe Pet Passport gives you the evidence to act early, before a tank mate is harmed.