A common pleco escapes by force, not by squeezing
A common pleco enclosure becomes escape-proof when it resists pushing, not just jumping. The slender-fish advice about tiny slots still applies to a young pleco, but the adult's real threat is leverage: anything loose will eventually be tested by a strong fish in the dark.
Begin with a night-time mindset. Most pleco escapes are discovered in the morning, so judge every fitting by whether it survives eight unsupervised hours of nudging, not by whether it looks tidy at feeding time.
Secure a lid a pleco cannot lift
Securing a common pleco lid means adding weight, clips, or both. A substantial glass cover that sits in a frame is a good base, and retaining clips or a weighted light unit that rests properly across it removes the upward play a pleco exploits.
Hoods deserve the same scepticism as glass. A lightweight plastic hood with a feeding flap can be nudged if its hinges loosen, so test yours by pushing up from beneath at several points. If any corner gives easily to your hand, it gives to a determined pleco, and the fix is mechanical, not hopeful.
Close the plumbing routes a pleco can enter
Plumbing is the common pleco's second door, and it must be closed deliberately. Overflow boxes, weirs, and sump returns should be screened with guards coarse enough not to clog overnight yet fine enough to bar a pleco's head. A pleco that enters an overflow may survive trapped there, injured by the fall or stranded when levels drop, so inspect these spaces during weekly maintenance rather than assuming they stay empty.
External filter intakes need sturdy guards that cannot be knocked off. A pleco rasping algae from an intake can dislodge a loose cage, and the same fish may then investigate the open pipe. Push-fit every guard after cleaning, because a guard replaced carelessly during a filter service is the one most likely to be found on the substrate the next morning.
Arrange the inside so strength has nothing to use
Arranging a pleco tank for security means denying the fish leverage and launch points. Heavy driftwood and rockwork should sit stable on the base or on well-settled supports, never balanced where a burrowing, pushing pleco can topple them against the glass or pin itself beneath. A falling decor injury at night can drive frantic thrashing that ends at the lid, so stability is escape prevention as much as safety.
Leave swimming and turning room that matches the fish's size. A common pleco crammed into a tank it has outgrown rests against the glass, barges obstacles aside, and stresses constantly, all of which raise escape attempts. Honest sizing is therefore part of this answer: an enclosure can only be made truly secure when it is also large enough for the adult fish to behave normally inside it.
If a pleco gets out despite the defences
A common pleco found out of water should be handled promptly and calmly. Watch its breathing, skin, and fins over the following days, and seek an aquatic veterinarian if it stops feeding, develops damaged patches, or breathes hard after settling.
Then audit in the order the fish would: lid lift, rear strip, overflow guard, intake cage. Fix the actual failure and re-test it by hand. Keeping that audit trail with the pleco's details in KinsLe, where the Lost Pet Alert System stands ready if this strong fish is ever missing from its tank, means the next keeper of the routine inherits the knowledge, not just the tank.