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The wen may not actually be shrinking. The fish may be getting bigger while the wen is staying the same size, making the wen seem like its shrinking
I was thinking the same thing as this was happening to one of my Redcap Orandas! Nice big wen, then it started to look smaller and smaller and then I realized the Redcap had grown in lenght quite a lot!
As far as wen growth, all the above members have given you great suggestions with different foods. High protein is a good way to go, but also genetics will play a big roll in how big the wen grows and also the care the fish got before you got it. I have also read that some feed their goldfish eggs to promote growth and have also seen tofu listed. Also gel food can be made that are high in protein with tuna and sadines.