If the fountain is in addition to the filter, then it may be not be needed. A lot depends on the size of your pond and how it is set up. A really big pond may not need any aeration, or much filtration at all. A really tiny pond will need all the aeration it can get, as the surface area won't be large enough to oxygenate the water for fish (assuming you have fish in there). A mid-sized pond of, say 2,000 liters could probably get by quite well with just the moving water from the filter as aeration.
For example, my set up is around 4,000 liters plus-minus, and has no filtration other than the plants in the pond, and no aeration aside from a stream and waterfall. It will eventually have a wetland filter, but that won't add much in terms of aeration.
When the weather is really hot, more aeration is needed, regardless of the pond's size, particularly if the air is still.
I don't live in an area that gets snow or freezes (at least not anymore) but others on the forum can better advise you there.