in one of my smaller pond, would a pump actually cool the water or warm it, if it was moving the water as a stream would?

I'll guess that it wont have any negligible affect, but lets wait until someone who knows what they're talking about chimes in.....

Whoo Whee. This a classic "it depends".

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.

ok, the small pond in 5 ft deep, does that say anything?

Well, you probably won't see a thermocline set up. depending on the ambient temp, I'd be more worried about it warming up the water too much.

I tried aerating my pond only when air temps were 68° and below, but the upper warm water mixed with the lower, cooler water and killed the fish.

Well I am using a pump to filter and add air to my 300 gallon tank. It is much cooler outside than what my water is.

Will the pump setup up like a fountain or waterfall make the temp drop faster from the cool air outside than if I stop it.

I need to know asap as my water is 60 now and I do not want it colder than that, but it is 48 outside.

Should I continue to leave the pump on or turn it off to keep the water as warm as possible until the sun warms it again tomorrow.

It has a few Tilapia in it right now, that is why I need it warm, and I do not have a heater at this time.

Any other ideas to keep it warm til these cool days leave for good.

Thanks

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)