These people at "Pets Paradise', as the old saying goes, 'Saw you coming'.
Thermo-Pond manufactures a 250 Watt unit that retails for about $65.00 and a 750 Watt that retails for about $70.00. These are made for ponds and are thermostatically controlled. In fact, the 1500 Watt that you purchased may be a Thermo-Pond product, as they do list a 1500 Watt unit. These should be easily found on-line. If not, contact me and I can have one drop shipped to you.
Does the unit that you bought have a pre-set thermostat? If so, and it is pre-set to maintain just above freezing temps, it may work OK. If not you will see substantial increases in you electric bill plus run the risk of upsetting the balance of the entire pond.
"Everybody is ignorant, only on different subjects." - Will Rogers