No, Siamese algae eaters are not aggressive. For the most part, Siamese algae eaters are peaceful and gentle creatures. They’re also unlikely to harass their tank mates. For this reason, these energetic bottom-dwellers are compatible with a wide variety of tank mates.
With that said, these bottom-dwellers can be somewhat territorial. If they claim an area of the tank, eventually, territorial disputes might break out. To prevent this, keeping a single Siamese algae eater or keeping them within a group of five or larger is best.
This will replicate their natural living condition and encourage a harmonious tank dynamic.