Why is my plant yellow at the top?

That looks like nute lockout to me. Immobile nutrient issue as it's yellowing from the top down. I personally don't use biobizz so take what im saying with a grain of salt. If feeding high amounts every time, with cause lockout. You are adding a lot of things every time you water/feed. Ppm is not standard, so most use ec. But if your on a 800ppm that is on a 500ppm scale to 1 ec, you are feeding at 1.3ec. Not bad, but gonna need to go to a feed one day, then water next time schedule at least. Otherwise could cause nute lockout. If it is nute lockout, you're gonna need to flush to get it back right. Over feeding can leave behind unused nutes. That can lead to salt build up in the pot. If it's only minor, using just ph'd water for the next couple feedings should help correct. If not then you'll need to flush. 3 times the amount of water as the pot to wash away any salt deposits. Then feed with normal nute feed to give it some nutes back. If it's progressing fast, I would flush. Again I don't use the nutes you are using, so I may he off base here. Hopefully someone with more experience will chime in and tell you if I'm correct or not. Just what it looks like to me from your pics and diary. Best of luck, hope this helps, Happy Growing!!