Hi Larry,
I have seen two solar options for aerating small ponds that have worked. I have found the solar fountains for waterfalls or fountains are cheaper to buy than an actual aerator that has an air pump and diffusers.
If you are looking for a solar air pump with tubing and diffusers the system I use is a small pump actually designed for bait tanks but they work well in any sized shallow (2 foot max diffuser depth) fish pond. The system is very cheap, only $25, but I've found that one system is only good for 50 square feet. So in your case you would need 5 of the units which would cost around $125. This isn't bad because I have seen solar aeration systems going for beyond $3000. I have a few ponds where I buy a bunch of these cheap aerators, and keep a few on standby, and have had great results. I do like to recycle but these things are almost disposable aerators at $25 each. We know of a very good solar pump if you want to build your own system!
If you use this bubbler type of solar aerator you should space the five or so units in different areas around the pond with the panels pointing in different directions. The pumps only work effectively when the sun is on the panels which is why it is better to have five or six units around a pond so that there is always at least one getting good sunlight.
If you want to use a solar fountain that just sprays water in to the air from a submersible pump you can actually have a very nice solar fountain that helps aerate and really circulates quite well which is important to maintain good beneficial bacteria healthy for your koi. T
he solar fountain I use is an 18V system with two panels and a really nice spray pattern delivered from the pump. The cost is around $280. It has two solar panels and what I like about it most is a three-year warranty and I have yet to install one where there were problems with the units. For a small Koi pond they are great and the multiple nozzles on the fountain mean you can really adjust to suit your pond. The system is really designed more for a pond like yours although if you don't want the fountain spray on your pond then the first option may be preferable.
Given the volume of your pond and doing the calculation makes me think that I would place two in the pond due to the volume of your pond. Place the panels of each pump at slightly different angles/directions to ensure a more even supply of pumping. Is an electric pump not an option? A single electric pump, with fountain attachments included, would be cheaper than two solar fountains and would pump more water. The electric fountain pumps we use are here. They do much higher flow for less cost...but they need electricity so sometimes solar is the only way to go.
The solar system might be the best back-up beyond a constant electric system. Another back-up is to use a a product that is designed for algae control but actually works partly by releasing oxygen. We sell a product called Granular Algaecide (Non-Copper/Non-Synthetic) For Algae Control that is an algaecide and not at all harmful for fish or plants or pets or people and one of the side-effects of this granular algaecide is that oxygen is released. I have a few clients who use it during hot weather when the fish are gasping. While I prefer to use aeration and shade for my fish ponds the product can help in emergencies. You can find more info and buy the product here: http://www.thepondreport.com/pond-algaecide-natural-bacteria.shtml
