Hey there @Legenda, those are good questions.

For the first question, it's up to you. If you enjoyed having double harvests and you've got more fertilizer to keep doing it, then replanting with a new fertlizer would be the way to go. But if you don't mind having normal harvests, then you can leave it growing if you want.

And "Depleted" just means that the fertilizer is done, so you won't have anymore double harvests.

Let me know if anything I said was confusing, or if you have more questions! ^^

Two plants I can think of which are great for the Quadruple Harvest Fertilizer are the Lucid Evolvulus Flower and the Tricolor Flower Petals. Those take extremely long to produce, and their normal harvest is only 1.

I don't use the Double Harvest Fertilizer enough to know which plants are good for it, but since it works for 3 harvests, I'd guess that a plant which produces a lot each harvest might be good for it -- like Voxel Fruits, Pixpeppers, Vividus Fruits, and so on.

I hope that helps a bit! Let me know if you have more questions :)