Don't throw it in the trash. That's really terrible. I think it says a lot about how you really feel about your fish if you just toss them in the trash. They aren't trash. Maybe it's just me, but I love my fish very dearly. I feel close to them. I know it's in my head, but I do, and it means a lot to me. I agree with kevinfleming, seal the little guy in a freezer bag, and get a nice little box. You could go to a thrift store and they always have little boxes like that. Then go to a park, or near a river, or something, and bury the fish. I live in an apartment so I can't bury mine either. But this is what I do, I freeze them and then bury them in a nearby wooded area. From the Earth, back to the Earth.
I like to treat my fish with respect, and I imagine their energy wafting up and back into the universe to be re-used elsewhere.
I really hope it works out for you, and I'm sorry about your fish.