Q.Houseplant gardenia
New growth leaves are yellow what should I do?

Too much water, pH balance being off and soil deficiencies can all contribute to yellowing leaves on Gardenia bushes. Since only the new growth is showing up yellow, try to think if something has recently changed in the plant's environment. This article may help you identify what's causing the yellow leaves and then correct it:
https://www.gardeningknowhow.com/edible/helping-a-gardenia-bush-with-yellow-leaves.htm