For the purposes of this discussion I am going to assume that you have lots of money to spend on electricity and heaters. Without that assumption this seems unfeasible to me.

For some serious BTUs you can use a regular domestic hot water heater. This can provide many times more heat than your aquarium heaters. Since you are already pumping the water back and forth anyway, you just route it through your heater. You will want to put in a thermostat that can be set to 65 degrees, or at minimum an on/off switch so you can manually control the heater.

You can estimate the wattage needed by looking at how far under 65 degrees you were on your 35 degree night, and how much your 900 watt heater warmed the water over what you would have expected without a heater. Use the ratio of these two, square it, and multiply by 900 watts. Don't be surprised if this is a large number.

I corrected the last paragraph. Sorry.

You can estimate the wattage needed by looking at how far under 65 degrees you were on your 35 degree night without the heater, and how much your 900 watt heater warmed the water over what you would have expected without a heater. Use the ratio of these two, square it, and multiply by 900 watts. Don't be surprised if this is a large number.

Hi
The 35 was for two hours on the 3rd
of this month and the daytime high was 65. Low since has been 53 with highs in the eighties.
How would I arrive at a ratio?? Besides wouldn't the water fall to whatever the air temp was?? Why would it stay above air temp??
I just started heating the water of the entire system last year.I just put styrofoam on the tank and used a 300
watt heater.Has never failed to keep the tank at 72 except without the styrofoam with temps below 40. even then it stayed above 65. I figure 3 times would be enough??
How can I arrive at a ratio without an example??
Of course I could always shut down the circulation system and use bubble wrap for the pool and add a space heater for the air. But I figure that I could just add X number of watts and do it all?? Thanks for the help!!! gary

Water has lots of thermal mass. Presumably the pond makes contact with the ground, which also has thermal mass. This thermal mass prevents the water from reaching the nighttime low temperature in the winter and prevents it from reaching the daytime high temperature in the summer.

Calculating heat loss is a very complex task even for an engineer, so the best way to start is make as many simplifying assumptions as possible. Trying to compare a 35 degree night with a 53 degree night is adding complexity to an already complex situation. Initially you gave a target temperature of 65 degrees, a nighttime temperature of 35 degrees, and a heat input of 900 watts. If you want to know how many watts you need to achieve your target temperature I need to know how much your heater warmed the water on a given night, and then compare the temperature on another similar night without the heater running. This assumes the starting water temperature is the same, the ground temperature is the same, and the two nights are the same in temperature, humidity, wind, and cloud cover. If you don't make these assumptions, the calculation is way too complex for me.

Hi
Well let's approach this from a different direction lol
It could be years before this happens again.
At 35 the tank dropped to 56 though the thermostat was set at 72 I added the insulation and shut off the waterfall and the tank returned to 72 within one hour but the pool was in full sunlight with bubblewrap on the surface which may have been a bigger factor than the heater??
Obviosly,only the 900 watts was having a strong effect on the temp during the night. So was working just not enough.lol So you don't think another 900 watts or whatever would have mattered??
I divided the heaters into 300 watts in 3 locations in case a themostat fails. I can easily step these up to 2500 watts each or add more 300 or whatever. Just seemed like way too much considering the most the water has ever need to be heated was 40 degrees above air. Large heaters would require sheathing to keep animals away naturally which would slow circulation.
Maybe I should just add another 300 watts for this year as I feel winter is over at least for 30's. Sooner or later I'll arrive at a balance?? the heat loss from 30's is obviously much greater in 30's as opposed to 40's.
A big danger with any heating system in this location is overheating as the temp frequently jumps 25 degrees when the sun rises... I thank you very much for the help. gary