I have a whole fish room minus the lights on a 15 amp breaker. That's a number of 50 watt heaters, 2x300, 150 and a 250. Two large hobs, mag 18, xp4 and two air pumps. Even with 3 more heaters I didnt trip the breaker.
I only had to rewire for two 800w heaters that pull 9amps a piece. Now my 450g has three dedicated circuits. 2x30 and 1x15 for lights. Those circuits will probably also run another 800g or so soon. I doubt you would have any problems. Just try it and keep an eye on things. If you overload it, you trip the circuit and you've got to rethink.