I'm not an expert but you may have too many fish for too little water. May need to take some out. It is getting hot here in Kansas and your problem will likely only worsen. I think you are heading down the right path with aeration.
What I am using is probably not a long term solution but I have been using cheap linear air pumps for a couple years now. Eventually I will probably replace with a good pump but they seem to last about a season running continuous (8-9 months) before needing a fairly cheap rebuild (12-15 dollars)and would be more than adequate for your pond depth and size. I bought mine at Webb Water Gardens and the pump brand is Pondmaster AP 100. They are available at lots of places by doing a web search.
There are a lot better pumps but for around 150 dollars this would get you by for this season at least. I run 4 of them 24-7. Some day I will likely put in a more expensive pump but these are doing the job. They are not any good for anything over about 9' max depth. The air flow drops to almost nothing any deeper. But I am running them at 8-9' with good water flow created.
Once I look it up, I will edit this post and add a link to my threads where I show pictures and talk about my system.
4 Pondmaster pumps in the housing I built Look at picture 003
Temporary setup Shows my initial setup testing to see if the cheap pumps would work. The diffusers I knew would be permanent but I ran one of the Pondmaster AP100 pumps for a while the first fall just to see if they were effective before buying more. Look at picture 006 and read the description of the pump
single diffuser in 1/10th acre pond This shows a picture of a single Matala 9" diffuser running in a 1/10th acre sediment pond. It is taking a little more than half the air from one of the AP100 pumps with the rest going to another single diffuser in my 1/20th acre forage pond. The next couple posts below this one shows some more pictures of the setup.
Some additional info Just a few more pictures and description, this time in my 1/20th acre forage pond.
Like I said, the Pandmaster AP100 pumps are not the best long term solution for aeration, but it is a rather inexpensive way to get a small shallow system up and running to see if it is something you want to continue to do. They have worked for me, but as they wear out I likely will put in a single big pump for the 4 acres total of water I am aerating right now. Then just manifold out to the diffusers.
Edit: I see they are not as cheap as they used to be. Now a couple hundred dollars. I think I paid around 135 for mine when on sale. I do not want to put a link to it because they are not an advertiser on Pond Boss but if you do a search for "webb water garden pondmaster ap100" it will come up. At that price you may actually find a better solution than the AP100 for inexpensive.
