It all depends on if the water is stratified, the ambient temp, pond temp, etc., etc.
For instance. If the pond did have a thermocline, and you pumped cool bottom water over an artificial stream and the ambient temp was greater than the water temp, it'd warm it up.
Since you have electricity near the tank, I'd run out and get a 300 or 400 watt aquarium heater as an insurance policy. While it won't warm up the whole tank, it will have a pocket of warmer around it. If you only have a few Tilapia in the tank I'd shut off the pump during the coldest part of the day until the temps warm up a bit, then go back to 24/7 operation. Use a thermometer to monitor the water temp. to give you an idea how long you can run the pump.
I almost got one off ebay as I figured it may work for what I needed, I had also considered a bucket heater as well.
Then it warmed up for a few weeks and was nice, now it decides to cool down below normal and I do not have one. I figure the time I get one the nice temps should be back.
I did find a small 100 watt one and went ahead this morning and put it in.
I turned off the pump last night and checked the water this morning and it was still 60. It hit 39 air temp for a low last night so I guess turning off the pump helped.
Should I consider some more air of some sort, or mechanical filter?
Well, if the fish are going to be in the 300 gallon tank for any length of time, I'd either plan on using some sort of pond filter to help minimize the waste in the water (and ammonia) or I'd plan on doing water changes every other day or so. (and make sure the water going in is warm enough for them)