I came home from work today to find my "mostly invisible plecostomus" was now a visibly dead plecostomus at the bottom of the tank. It was three and a half inches long at time of death, no signs of fungus, ick, bloating, starvation or any other known problem. I'm fairly certain the red-finned shark killed it.
The rear area of the "barb barrio" is fully planted with water wisteria plants. The "mostly invisible plecostomus" was always hidden behind a plant, under the heater and bubble tube; it simply laid there all day, it's sucker mouth attached to a smooth rock. This plecostomus was the most territorial and aggressive of the two I owned when all my fish were in a 29 gallon tank. The left rear corner of the tank was dedicated to the red-finned shark as it's "cave"; the place it will defend and patrol. Wednesday, the day before he death, I noticed the plecostomus was not in his usually spot at the rear middle part of the tank, it was attached to the glass at the front left side of the tank. I assume that when the shark was patrolling the rear of the tank behind the plants, it chased away the plecostomus.
The only other causes I can think of would be either the shark has some sort of disease that is not noticeably recognizable or the onion plant had something on it the plecostomus ingested and died from. I'm ruling out starvation. Two algae wafers were dropped in the tank every other day, the same amount as before with the same fish plus an extra plecostomus. I'm fairly certain this plecostomus was digging up wisteria plants and stripping the bottoms of those plants.
In any case, it sucks to lose a large fish. "Little Plec" started as a Wal-Mart purchase to clean up algae in my tiger barb fry tank. Starting at just over an inch "Little Plec" grew quickly to over two inches, earning it a spot in the big tank. It had black stripes on it's tail, unlike all the other plecos I've had. It was like most older plecos, though. They don't eat algae, they uproot and strip plants, and you never see them until they come out under darkness for their algae wafer or zucchini wedge.