Hello guys, I noticed this on my intake right now. This is in my little 3.5 gallon QT, last fish I had in there were a few guppies 2 weeks ago. Tank has been empty since but I kept it running and drop in fish flakes every few days and half an algae wafer few days ago. Honestly haven’t tested the water since I moved the guppies but it was perfect while they were in there. Tank is set at 78. Filter has been on. No tank light has been on but there’s ambient light for half the day.