that's a very nice idea you have there and i like it a lot. I have a few things to mention though
1) going along with the tamed fish idea in the op, have a way to make sure they don't despawn, and then be able to put them in regular water. That way, i can make a koi pond :biggrin.gif:
2) if you for whatever reason find it hard to program them to stay in regular water, have the glass tanks be able to be merged for irregular shapes, for the same idea as the koi pond.
3) if you add that many koi to the game (i know you're only starting with two, but if you add all) i would personally require a properties file that had spawn rates and the ability to turn off spawning (and change any ids created)
4) might i suggest other fish? such as bass or goldfish or the like?
but very nice work, i wish to become as good at java as you must be to be taking on this ordeal.
How about a tiger koi? It could be orange, white, or a golden-ish color and has black stripes (the golden is just darker gold stripes, its a golden tabby)
The eye color for the white one would be a grey, the orange brown and the golden yellow. The orange is not as rare as the white, and the white isn't as rare as the golden tabby. They could spawn in moderately deep water (basically shallow-ish), and the orange tiger koi would take about 6 pellets, the white 7, and the golden tabby 8. And if this is even possible, make them spawn more often in rain forest biomes?
And maybe RED mushroom pellets? Since red mushrooms are a bit more rare, it would take less time to smelt and (perhaps) give the value of two brown pellets.