For materials--shoes, stoves, ladders, cell phones, just about any object imagineable, I have one word for you son: Barbie. Actually kid toys of any sort. What's even better is that you want them already broken! since broken things which are hard/expensive to dispose of properly tend to be the objects thrown into waterways. Stolen items or stuff used in crime is also tossed into rivers: Tools. Guns, if you want to be really creepy--boys dolls, oops I mean "action figures" are good for these. A couple of piles of unidentifiable matter can be freehanded out of play-doh for that added post-industrial vibe.
I bet if you ask around people with kids or younger siblings, or daycare centers, or yard sales, you could cover the bottom of a tank 10 times the size. Junk Substrate!
For fish you want "ugly." Dull. Icky. Female guppies. Baby goldfish before they've gotten their colors: they look like carp. Catfish and other bottom feeders. A pleco if you can find one small enough. Again these should be anywhere from cheap to free if you seek out culls. A couple-three ghost shrimp would add atmosphere but not much color, which is what you want. Of course you have to figure out what to do with them after the art project is over with but you knew that already.
Definitely looking forward to pictures. Is this for a class, or a contest, or your own personal artistic expression? Very cool idea in any case.