a better explanation from Sensei
Probably not the poo so much as just oxygenated water. If there was any significant ammonia in the water the fish would be unwell I think. Ammonia, and salt, tends to promote the development of various copper salts which give us the common green/blue types of 'copper rust'.
With your pieces I suspect the colour is coming almost entirely from cuprite (Cu2O, basically 2 copper atoms binding with one oxygen atom) which is red. Other alloy components, like zinc in this case, can become oxides of themselves, zinc oxide is white, and thereby serve to alter the red colour we see from the cuprite. There might be some traces of sulphides present which would also further modify the colour we 'see'. Copper alloys are not perfect mixtures, more like a cake with raisins in it. You can imagine a field of red cuprite with spots of white zinc oxide scattered about. We perceive a colour as one tone but it's a bitlike a TV image in that it's made up of only three colours, red, green and blue.
A further 'modifier' to the wavelengths of light reflecting off the patina surface is the actual physical structure of that patina layer. Not so much the mechanical polish, although that is a part of it, but more the speed and time scale at which the patna grew. This effects the surface structure which in turn effect how light behaves when it hits it and is reflected. Less 'artfully' created patina can often have a 'stain' like layer form which inhibits further patina grows and leaves a slight translucent interference layer instead. This can look a bit like petrol/or gas on water.
It's obviously much more involved so I'm just trying to describe what's going on as simply as possible.
Hope some of the above helps. Feel free to quote me :-)