Like all loaches, the species develops an hierarchy. What you see is certainly not at all unusual, and need not end in disaster. Generally peaceful, in its habitat this loach lives in large groups; males will "defend" their territory (selected on the basis of available food) by "topping." When a second male invades another's territory, one fish tries to cover the other in a test of strength, termed "topping." There is rarely any damage inflicted, and one fish eventually retreats. The dominant males claim the best algae areas as their territory, while females tend to congregate in other areas. This fish must be in groups of at least six, or it can be very withdrawn and inactive. You might consider two or three more. They need stronger water current, and cooler than normal tropical temperatures, and lots of algae-covered smooth rocks. River rock pieces placed in a bucket of water in direct sunlight will soon supply this, and they can be replaced as needed.