Plecos are "protected" by several features:
- They're nocturnal, and most our agressive fishes are diurnal, what makes them naturally avoid the agression;
- They have an armour of bony plates, not scales. their vulnerable parts are their bellys and eyes;
- Several species bear spines, some are EXTREMELY spiny. This, and the pectoral and dorsal fin "spikes" (enlarged first rays) make it difficult to swallow a pleco;
- They heal fast, recovering from bites and scratchs easily;
- Most species are cryptic, camouflated, or keep hiding under the tank furniture, making it difficult to find them and/or get them;
- Plecos are VERY fast swimmers, especially when fleeing away from a menace. They seek for shelter, and usually become out of reach quickly.
Best regards.
Daniel.