We planted it, and it is now nearly 9 feet tall in a pot in our solarium. The plant now has spider mites, but it is very difficult to spray a 9-foot tree while it is indoors. Mites love citrus plants but hate high humidity. Mist your grapefruit often if you keep it indoors through the summer, and occasionally take it outdoors for a quick simulated thunderstorm from your garden hose. If none of these measures keeps the mites under control, you'll have to spray them.
Nothing is more effective in killing mites than horticultural oil.
Oil kills all stages of mites from egg to adult, so one very thorough spraying might be all that is needed. Use it at half the rate specified on the label. Allow the plant to dry before returning it indoors. If you move the tree outdoors during the summer, it is best to spray it with horticultural oil on the day you return it to the solarium because it will likely have a few mites on it that have survived summer rains. In a protected indoor environment, these hitchhikers can quickly explode into mites by the thousand.