I have jus bought a motoro stingray and a teacup stingray. I have them in a 180 gallon freshwater tank. I noticed the motoro has a quarter size brown spot on his underneath ? Does anyone have any idea what it could be? My water is excellent. Ph might be a little high but other than that it's good. I have a fluval fx5 filter and temp is 78.i bought it 4 days ago. Teacup is doing really good!! They are both eating well.