Large goldfish with long fins combined with a strong powerhead equals problems, no matter how healthy the fish. I know this does not make sense to most people, but if you have ever kept these fish, you know what I am talking about. I recently took both powerheads out of my tank, because one tilted just a bit off the riser tube and opened up a small gap between the intake of the powerhead and the riser tube. Sure enough, as I was getting ready to get in there and fix it, one of my fish got her belly too close. She pulled herself loose quickly, but she lost a few scales. I can only hope that she did not get damaged internally.
When it comes to mounting a powerhead to the glass, are there brackets (or something) sold for this purpose? If I do put one back in, with a sponge over the intake, I would like to be able to move it to a new location periodically.