Here are the 2 flatheads I have.( idk why I thought one was a bullhead, turns out it's a flatty) they are doing well in the 450 View attachment 1030147View attachment 1030148
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Any thoughts on putting a couple dozen blue gill in the 5500g? They would be able to provide food for your flathead, and a couple dozen might be able to reach self sustainability levels.
Side note: Whats wrong with Madtoms? They should be great for your 40g.