First step is to estimate what your load will be. If you put two 500W heaters on the tank, it will definitely be the biggest pull. Will you use sump or canister filters. Estimate the load on those. If you use LED lighting, the load will be minimal. Will you have a power head or two? Total up all of your watts before you call your electrician to let him know what kind of load you are expecting to add. Since the receptacle outlets themselves are typically only 15 amps rated, you will not want to put more than 80% of the 15A on there. That means try not to have more than 1440 watts (80% of 15 x 120V = 1440W) of load on any one outlet. Next you will need to know what other loads are plugged into the same circuit. Go around the room and find out what is plugged into those outlets. Total it all up. When you have the total, contact the electrician and explain what you're trying to do. All in all, if you have a spare breaker, it would always be best to have the tank and load on it's own breaker so that vacuums and other loads plugged in nearby don't cause problems with your tank, but that's an expense you will have to consider and the electrician can help you with that.