Growing up means claiming ground
The pleco that spent its youth drifting between hiding spots is running a juvenile strategy: stay invisible, stay alive. As plecos mature — and as males mature in particular — the strategy inverts. In the wild, a male pleco's cave is his breeding chamber: he claims one, keeps it clean, and fans eggs inside it, and a male with no defensible cave simply does not breed. Hormones promote your fish from squatter to landlord. The chasing, flaring and barging you now see at the cave mouth is that landlord behaviour arriving on schedule, and it intensifies with size because each growth spurt makes him both more capable of holding the cave and more invested in it.
One cave is a prize worth fighting for
Owners are often puzzled that the war is over one cave when the tank has three. Look at the caves from the pleco's side. The prized one is usually the snug fit — plecos like a cave they can wedge into, touching the walls — placed where gentle current delivers food scraps, with a single entrance he can block with his own body. The spare caves are too roomy, too exposed, or face the room's traffic, so as far as he is concerned the tank contains exactly one cave and two decorations. Other bottom-dwellers get chased because they blunder into the doorway he guards; fish chased hard at that one entrance may barely be noticed anywhere else in the tank.
Give the tank more addresses
The reliable fix is supply. Keep more suitable caves than you have cave-wanting fish, and make them genuinely comparable: snug tubes, coconut caves or slate lean-tos in different corners, at different distances from the filter outflow, each with one entrance. Space matters as much as count — two caves side by side are one neighbourhood to defend, while caves at opposite ends of the tank are separate properties. Rearranging the existing decor during a water change also resets the map; a pleco returning to shuffled territory spends his energy re-surveying instead of defending. Given a real choice of addresses, most plecos settle into the best one and tolerate neighbours in the rest.
Feeding the peace at lights-out
Cave wars flare hardest around food. A pleco who eats at the cave mouth comes to treat every sinking wafer's landing zone as part of the estate, and tankmates that arrive for a share get charged. Feed after lights-out, when plecos naturally forage and shy tankmates move freely, and drop food in two places at once — one serving near his cave, one at the far end — so nobody has to cross the guarded doorway to eat. Driftwood matters here too: a pleco with wood to rasp on spends long contented hours grazing instead of patrolling, and a well-fed landlord is measurably more relaxed than a hungry one.
When guarding turns into damage
Charging and posturing at a cave entrance are normal; injuries are not. Take action — more caves, a decor reset, or rehoming one fish — if tankmates show torn fins or scraped flanks, if a smaller bottom-dweller stops coming out to eat, or if the pleco starts ranging the whole tank to attack rather than defending his doorway. A fish being starved by exclusion loses condition fast and quietly. Logging which fish get chased, and where, in KinsLe through Care Teams shows whether your new cave arrangement actually ended the war or just moved it to a different corner.