I’d clean with either citric acid or vinegar. As far as removing the pump, the little elbow at the top where the tube connects to the outflow can be disconnected and the use the tube or power cord to pull it out, it’s not glued or screwed together.
Edit: I’d also get rid of all the media and sand, you don’t know if they ever used copper in the tank and it’s just not worth the risk. I’d replace the plastic bioballs or whatever they’re called with some marine pure blocks/balls and run some Chemipure blue. It looks like they scraped out a window in the back middle chamber, so you could grow some chaeto or macroalgae back there (I’m not going to call it a refugium, growing some macroalgae does not a refugium make, it’s more of an algae scrubber), just get a light that you can put up against the back glass.
