When I ran my 29 with a 10 gallon sump and diy overflow, I had some small dragon gobies and the smaller one ended up swimming his way down through the 1" pvc overflow into my sump, twice. Lol
I had a bn pleco swim up the intake tube of my Penguin 350. The strainer had gotten knocked off and the little guy swam up it and got stuck at the "U". I had to cut open the intake tube and pull him out by his tail but he was stuck in there for a couple days before I was able to get him out. He did this not once...but TWICE within a few months!