With yellow leaves you are supposed to trim them. Cuz all the plants energy will go towards fixing the leaves instead of growing. So do this, it should help.
Trim the yellow leaves
Get flourish comprehensive (MUCH better than api)
Also depends on your budget, you could get flourish, potassium, excel, and as many of the of the flourish line as you can. The more the better. But again definitely not a necessity.
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