You do not really have a pond, technically it is a wetland with those maximum depths of 3-4ft. For shallow ponds such as yours, managing it as a 'regular' pond will be a big, big challenge.
For aeration you do not need a compressor that produces more air pressure (psi) than 3-4psi. I would not use a water pump fountain style since it costs more per hr to run those if the fountain moves an appreciable amount of water similar to a bottom diffuser.
An electrically efficient type compressor would be one of the linear style air pumps which are pretty good for shallow ponds (<6ft). Even if you run the aerator continuously and even add more of them, be advised that an aerator/s will not eliminate your algae/weed problems and could actually create better conditions for plants to grow. Working to maintain a relatively 'weed' free pond of 0.2-0.5ac will be a big challenge if you don't use lots "chemicals". Shallow water almost always results in naturally weedy ponds unless special conditions are used. Generally the weed problems in ponds always occur in the shallower waters not in the deeper middle area.
It would be a very good idea if you read through this long detailed thread as good homework and background about pond aeration.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92494#Post92494Note the posts about DIY aeration which mention some economical aeration systems. Note the compressors of the DIY systems may not last as long as the 'standard' compressors for pond/lake aeration. Members are learning this. If you are handy, I found that the diaphragm of EcoPlus pumps can be rebuilt-replaced without buying a new pump. Rebuild kits are sometimes not available and one has to fabricate their own new diaphragm -(search YouTube video).
Long term IMO, I would research the rebuilding/redigging of this pond to a depth of 10-14ft with steep sides & minimal shallow water. A smaller deeper pond is much easier to manage than a larger shallower pond.
