If the air temperature is constant... and the water temperature fluctuates that much (4*F)... then I would highly question the quality of the heaters...
My Stealth 250 W keeps my 125 gal no lower than 80, no higher than 81 (per both digital and glass thermometers) whether the house is 75 in the summer or 66 at night in the winter...
But if your still not willing to trust two Stealth 250W heaters... I would suggest considering two 400W Hydor, as Hydor seems to be a fairly trustworth heater manufacturer (I've heard of people using them but have not heard of people complaining about them.)
And you are right... typical pond deicers do not heat the water... they vibrate and the motion prevents the water from icing over in the small area around the deicer. The water movement it creates also encourages a gaseous exchange.
My Stealth 250 W keeps my 125 gal no lower than 80, no higher than 81 (per both digital and glass thermometers) whether the house is 75 in the summer or 66 at night in the winter...
But if your still not willing to trust two Stealth 250W heaters... I would suggest considering two 400W Hydor, as Hydor seems to be a fairly trustworth heater manufacturer (I've heard of people using them but have not heard of people complaining about them.)
And you are right... typical pond deicers do not heat the water... they vibrate and the motion prevents the water from icing over in the small area around the deicer. The water movement it creates also encourages a gaseous exchange.