The six-inch fish sold at one inch
Siamese algae eaters are sold as tiny striped juveniles, and almost everything about their tank requirement comes from the adult they become: a streamlined, restless swimmer of about six inches that covers the whole tank, all day, investigating every surface. Thirty gallons is the floor for one, and it should be a long thirty with unbroken swimming runs rather than a tall cube of the same volume. A group — the better way to keep them — wants fifty-five gallons so their squabbles have somewhere to dissipate. A tall, narrow tank fails them even at the right gallon count, because what this fish actually uses, hour after hour, is length.
One, several, but rarely two or three
This species has a well-known middle-ground problem. A single fish is usually peaceful, grazing and minding its own business. A group of five or more spreads their chasing so thin it stays harmless, each fish distracted by the others. But two or three in a minimum tank can settle into a relentless pursuit of the weakest, who gets no rest and slowly loses condition behind the plants. Decide which keeping style you are committing to before the tank is chosen, because the minimum for a lone fish and the minimum for a group are different tanks entirely. Adults can also grow lazy about algae and bold about food, competing at the surface with a determination that surprises keepers who bought them as background staff. Their name promises algae control, and young fish do graze soft green algae and the film on leaves diligently — but they are omnivores that need vegetable matter and ordinary foods in their own right, and an adult expected to live on whatever the tank grows will simply start taking the other fishes' dinner instead.
Know the fish you are actually buying
The name gets attached to several look-alikes, and the distinction matters at buying size. A true siamese algae eater carries its black stripe unbroken through the fork of the tail, has a slightly down-turned sucker mouth with small barbels, transparent fins, and rests propped up on its fins rather than stuck flat to the glass. Fish sold under the same name that lack the stripe continuing into the tail will not graze the same algae. The unrelated Chinese algae eater is a different trap again: it grows larger, turns aggressive with age and latches onto the flanks of flat-bodied tankmates. Either impostor outgrows a minimum tank in temperament long before it does in length, so check the stripe before the fish goes in the bag.
A working crew needs a rota
Because a group of algae eaters is kept as a working crew, their care is a routine of small checks — who is eating, who is being chased, whether the weakest fish still reaches food before the others clear it — and those checks are easy to assume someone else has done. Listing them as assigned tasks in KinsLe Care Teams, each one ticked off with a timestamp when done, means a chased fish losing condition gets noticed by the household as a pattern in the record, not discovered late by one person.