This may be nothing, but I thought I should ask about it. I was watching my bristlenose pleco, and noticed his tail kept twitching to the side. It bends up at about a 45 degree angle for a moment, and then falls back down to its normal position. It always bends to the same side.
It's usually just the way the water current is positioned. They use their tails to balance themselves in the water (typically when stuck to a surface) and also to re-adjust a few millimeters at a time. They'll flick their tails to whichever side needed based on the direction of the current compared to their position. If they're staying in relatively the same position on a surface, they're going to keep flicking it in the same direction that helps them stabilize in the current.
Totally normal. My pleco and otos always do it when they're on the glass in an area with a current.