When you say "leaky," do you mean a seep, a drip, a flow, or a gush? If it is dripping or seeping, you might be able to remove the fitting and carefully add teflon tape to the threads or use a liquid thread sealant if you don't feel comfortable using tape. Without some type of thread sealant/lubricant, you just can't get fittings tight enough that they won't seep some fluid. It only takes a tiny 1/2 drop or less of seepage to cause a fitting to grow a dust "beard" and look awful. Luckily, it normally rinses off easily with just a blast of cold water.
So if you can't tighten the fitting, you might try removing and adding thread sealant. If it is just a tiny seep that can be rinsed off, you might be better off just living with it.