Answer: The tropical hibiscus should do well in containers with a good quality soil mix such as you used. Make sure the containers have drainage holes in the bottom. You would need to fertilize in addition to what is in the mix, yes. I would use a water soluble fertilizer formulated for acid loving plants with an analysis of 10-10-10 plus minors or similar proportions, read and follow the label instructions. Water to keep the soil damp like a wrung out sponge but not saturated or sopping wet. The plants are probably going through an adjustment period, it is not unusual for them to drop some foliage when they are moved and/or transplanted. This is especially true if the weather is cool or cloudy in the spring. Make sure they are in a full sun location with good air circulation.
In the fall, you can bring them inside to try to overwinter them. You can store them in a dark and cool location such as an unheated garage or basement, keep the soil just barely damp not bone dry, wait until temperatures are in the mid forties to bring them inside. Or, bring them in when temperatures are in the 50's and set them in a bright window; cut back on watering somewhat as they will use less once indoors. If it is bright enough they may continue blooming.
In late winter, trim them back a bit and then when the temperatures moderate set them outside again. Gradually acclimate them to the real sun and do not expose them to temperatures below 55 degrees. This may mean setting them outside on nice days and bringing them in at night.
I am not sure what spots you are seeing so this would be something to check with your retailer and/or county extension. If it is limited to just a couple of leaves I would pick them off and put them in the trash and see if that stops it. Enjoy your hibiscus!
Q&A Library Searching Tips