yellow leaf tips

hi guys i vegged my mj plant 4 weeks from seed i swithed to flower 5 days ago and just noticed yellow tips on 1 ov the 3 plants im growing in a dwc and nutes are around 500ppm im using canna aqua at the mo im still using canna aqua vega ive not switched to bloom nutes yet could this be the problem and advice welcome

hi just read your thread i have the exact same problem first week flower aqua vega ec is 1.9 ph 5.9 can you tell me how you sorted it please