Could anyone help me fix this

Hey there Fishcake, you're right that it's a pH fluctuation but the thing is it already happened and if you're already feeding at 6.5 the best you can do is feed at 6. but still it shouldnt be a problem. The problem happened then spread a bit but as long as it doesnt keep happening you're basically alright . Ideally you should be adding a "flower booster" like humic acid to fix some slight root problems on the problem plant but if you only follow a stricter wet/dry cycle the plant should recover in 10 days or less. Hope this helps ! 🚀