Dissimilar metals - stainless steel and nickel plated brass

I'm a fiberoptic lighting Technician installing stainless steel fixtures with nickel plated brass liquid-tight fittings that match the fixtures, in a fountain in New Jersey. I am concerned that these dissimilar metals will react with each other in the chlorinated water. Will there be a reaction? If so what can I do, short of using nylon fittings, to prevent it?

The short answer is yes, you will get galvanic as well as general corrosion action with those materials in that environment. The question is "at what rate?". There are a host of potential problems with those materials in that environment. I say 'potential' because if the component materials and system design are right, they could function perfectly well for many years. If not, they could be a disaster, failing in months. I've seen both. Try to find out if this system is in service at a similar environment elsewhere. If it is, call your counterpart at that facility and ask about performance. You probably don't want your project to be the initial trial for this system. Bottom line; potential serious problems. Check it out.