The April FOTM Contest Poll is open!
🏆 Click to vote! 🏆
They are bristlenosed plecosAre you certain it is cycled? At 6 weeks it may or may not be fully cycled. Readings of ammonia, nitrate, and nitrite will help determine.
What kind of plecos do you have? Some grow to massive sizes and even the smaller ones are too big for your tank.
Think you can post a picture of the tank and fish?
Or the tank is not cycled. The ammonia testing should answer that question. But until you have that answer do that big water changeI'd say with confidence that your test is inaccurate.