Thanks for all the info so far. I�m not too worried about having a flat bottom. Imo a v type bottom would be ok. I Think I would seine the pond every couple of weeks just to keep the amount of fish in it down so it doesn�t over populate. If I missed fish when I did that it wouldn�t be a big deal and when I drain it in the fall I would be able to get most if not all the rest of the fish out. I don�t think 4 or 5 foot deep will be near deep enough for a northern Iowa winter.
I think when I drain it in the fall I would build a cage and put the cage in the big pond for the winter and in spring restock the 20x20 pond with those fish. I�m not sure if that will work? I also think that a aerator would help keep the water quality better for the fish. I�m not sure if there is anything else I can do to keep the water quality in super good shape a barley bail maybe? I plan on putting a bunch of AG lime in the bottom when I build it. From what I have ready GSH eat FA so they alone should keep that in check but if they don�t I can always rake it out.
Thanks for all the help and thoughts so far there greatly help full.