A few things....
Poppins right on the calculations, it's fairly easy to figure out what load you can handle by multiplying voltage times amperage, thus your 15 amp at 120v yeilds 1800, for safety it's best to stay under 80% of that which is about 1440 ish. When running wire there are a few tricks, typically a 15 amp circuit calls for 12 guage wire and you have some options, if the slot in your breaker box is a stab slot you might be able to run a dual breaker thus doubling your circuits.
If it were me and the wiring was needed to be run I'd pull 12/3 wire (12 gauge 3 conductor with a 4th conductor as ground) and do a double breaker for two circuits. You can easily share the common and the other two conductors for your
hot on the circuits. Depending on length of run you can also go up to 20 amp on 12 gauge and double breakers are available in both 15 and 20 packages so thats something to look at as well!
if it were me I'd go with two circuits, ditch the 1000 watt heater and swap it out for a pair of 500's and then run a pump and heater on each breaker, that way you have redundancy in flow and temp! I'd also run each circuit on 20 amp and pop a nice 20 amp switch in right next to the outlet in the event I wanted to turn the outlets off and back on easily (tank feeding, etc). And of course
GFI.
