Angelfish are native to the Amazon River basin and prefer soft acidic waters. Plecos are native to South America and prefer hard, alkaline waters. These different water requirements make it difficult to keep both fish healthy in the same tank. Additionally, Angelfish can be aggressive towards other fish, including Plecos. The Pleco may also try to attach to the Angelfish, causing damage and stress to the Angelfish. For these reasons, it is not recommended to keep these two fish together in the same tank.