Ok, where to start with this one! Firstly how many watts are the heaters you need to run? And worst case scenario how many hours a day are they on for?
DISCLAIMER WARNING: THE FOLLOWING IS AN EXAMPLE ONLY AND ARE NOT DEFINITIVE VALUES FOR WHAT THE OP OF THIS THREAD REQUIRES!!!!! IF YOU WANT TO CREATE A SYSTEM LIKE THIS, A HELL OF A LOT OF MATHS IS INVOLVED IN THE SETUP AND I WOULD RECOMMEND GETTING A PROFESSIONAL TO DO THE WORK IF YOU ARE NOT COMPETANT IN ELECTRONICS DUE TO THE LEATHAL CURRENTS INVOLVED (IT'S CURRENT THAT KILLS, NOT VOLTAGE) AND THE HIGH ELECTRICAL FIRE RISK.
For the sake of this example lets keep it simple (!) and say you have a 100W heater and at it's most usage it's on for 24 hours a day. So that's 100W per hour, or 0.1KiloWatt Hours of consumption, which over 1 day would be 2.4KW hours. (heaters are connected to a thermostat to turn them off to avoid boiling your fish so this amount of usage isn't likely)
Now on most good car batteries it should say what the maximum current that can be drawn at once is and over how long it will last. This isn't a definitive value and isn't on all batteries, it'll vary with the battery's age and how it's been used (charged then emptied a few times will drastically reduce this.) When it's in a car the battery will constatntly be being topped up from the alternator (dynamo if you have a really old car!) so it never gets completely drained and lasts a lot longer.
A car battery produces a nominal 12 volts DC. Volts multiplied by Amps equals Watts. In a perfect system with no losses the 100W heater would need to draw about 8.5Amps from the battery while running. As the world isn't perfect, your heater needs to run off AC mains (I think that's 120V over there isn't it?) so your battery would need to be connected to a '12VDC to Mains AC' inverter rated at a minimum of 100Watts which will use more than 8.5Amps from your battery as there will be heat loss from the device.
Still with me?
Running your heaters like this to avoid plugging them into isn't an issue, how you go about keeping the batteries charged is the issue. The idea of fitting a turbine to your run down to a sump could work depending on how much your heater is actually on but i wouldn't recommend it. I have an 8,000litre per hour pump in my pond that draws just 80Watts of power to run. That's a huge amount of water but as perpetual motion is impossible, a water flow created at 80Watts would not be able to generate 100Watts through a turbine. In a perfect no loss world the turbine could only generate a maximum of 80W. If the 100W heater was only on for an average of 12 hours per day then as an average it'd just be drawing 50W. 80W generated in the turbine would be higher than a perfect 50W draw on the battery so it would work and keep the battery charged. A tank size requiring just 100W of heat would probably only be about 60litres, pumping 8,000litres per hour would be insane and i don't think there's any fish that'd be able to cope with that amount of flow rate!
The other options that i'd consider for this then are Wind or Solar power supplies. But i wouldn't just connect the battery to the supply when dead, i'd have battery/batteries connected to them all the time while it's also running the heater. That way there would be a constant charge going onto the battery while it was sunny or windy and the battery life should be extended. Also, in this configuration you could also look at it as the heater being run by the solar/wind supply and falling back onto the battery at night or when there's no wind.
The solar panel or wind turbine would also need to be properly sized to your usage.
Lets start with Solar! Say your solar panel can generate electricity while it's bright and sunny, and every single day of the year is bright and sunny, and on average over the year you have 12hours of bright sunny daylight per day then on an average over 24 hours your solar panel is only going to generate half what it's rated at. So in the perfect-no-loss-brightsunny world drawing 100Watts every hour, your solar panel would need to generate 200Watts to run the heater and charge the battery enough so the battery can run the heater when it's night time. The world being not perfect makes clouds and rain so your solar panel is not going to be working a peak efficiancy like that all day while it's not night so in reality it'd need to be a much much higher rating. I don't know what the % efficency is for solar panels in an average loction so don't know what rating would actually be required.
Same principle goes for wind power as it's not windy 100% of the time. I have a friend that used to work for a wind turbine company building 2MegaWatt turbines and even in the most windy offshore locations around the UK they'd only avarage about 25% efficency over the year. So depending on how windy your location is, to keep the 100W heater and battery going you'd need a minimum of a 400W turbine if your area was as high as 25% (locations are normally be less than 25%, so the Watt rating would need to be much higher.)
So going right back to the begining, your power source, be it solar or wind, to keep the battery charged and the 100W heater running for 24hours without there being an overall loss of power would need to generate 2.4KW every day if it was a perfect world.
Actual consumption, and thus the power you'd need to generate everyday to keep the system alive in the real world would have to be calculated from the values specified by the manufacturer of the power losses for each item making up the system. The system from start to finish being Wind turbine or Solar panel and it's associated circuitry (regulated 12V supply to charge battery), the battery itself which will have heat loss which uses up power, the 12V DC to AC Mains power inverter (lots of heat loss!) and finally the heater itself. The resulting value will be a lot lot lot lot more than 2.4KW Per day!
This being MFK, i'm guessing your total heater Wattage is a lot high than just a single 100W heater anyway! I commend you for wanting to try and save money and heat your tank through a green method, just be aware it's a lot more complicated than the initial thought would lead you to believe and the cost of some of these items at the size an MFKer requires can be very very expensive (UK wind turbines used to average cost £1 per watt generated to build, a 1KW turbine at 25% efficiency would only be good for an average 250W if you were lucky and cost you £1,000 to buy and build! Last i heard i think solar panels were even more expensive.)
I hope that's all of some help.
Edit: I probably should add before all that gets ripped to pieces that i've tried to keep it as simple and noddy as possible to try and make it easy to understand. And before anyone asks, i am an electronic engineer with lots of experience on high voltage and high current installations.
ill be going with solar i guess its about $150 to power my heater but its worth it in the end if you think about it
to power my pond it would take 1,200 watts but i was thinking of getting these 2 (very powerful heaters) that use 300 watts, it should be enough to keep my pond around 70F and at night am going to cut off criculation and have air going threw the pond so the pond can conserve heat. (this is being all plugged in mind you my grandpa agreed now...) but i still want to go somewhat green still!
is there a way of having wind+being pluged in at the same time? so it doest use so much engergy from the socket, so it would split it 50/50?
