Do plec's have a breeding routine like other spiecies? I have a 100 gallon tank and i have had my plec for about 6 months but got it from my sister who in turn got it from someone else because it out grew their tank, im not sure how old it is but is aprox 7 inches. i have acquired another plec today who is aprox 8-9 inches but is quite a bit broader than my original one. They have been chasing each other around then breaking off from each other then swimming in circles in a ying-yang type of position. I took this as a power struggle at first to determine the "head guy" sort of thing but now Im thinking it mite be that they are different sexes. My tank doesn't have any caves or dark hiding holes but i have a few orniments and two tree root looking items. any advice would be muchly appriciated, thanks joanne.
When you say "breeding routine" do you mean courting behavior? If that's what you mean then yes they do. Sorry to say what you are seeing is pure aggression and without looking at your set up I can't say for sure if they have adequate hiding spaces.
Large plecs are most commonly very aggressive toward one another. They should settle down soon enough and establish a pecking order. Your fish will both stake out a side of the tank and defend that side from any other large pleco. A tank smaller than 6 ft long will result in frequent territorial fighting as these big guys need some space.
Warning: plecs can do a lot of damage to each other so do keep an eye on them, one may need to be removed. Don't let the sucker mouth fool you, I've had plecs literally rip most of the exterior flesh off of the other plec over territorial fighting. The skin was just gone! These plecs were only 3-4 inches so larger ones are more than capable of the same thing.
Courting behavior most commonly involves the male hanging mostly out of his cave fanning his tail at the female. The female when ready to mate will start testing the male to see if he will let her in the cave. Over the course of a couple of days she will repeatedly try to enter the cave until the male finally gives in and lets her in, them pins her in the back of the cave and at the moment breeding occurs.