Corals are far less of a problem as it is not common practice to use cyanide or dynamite to harvest them ( crowbars occasionally, which is not good), but fish collection practices have longer-term, farther reaching impacts on habitat that impact more than the target species.
You are right there is a small fraction of the desired fish species available as aquacultued, but there are locations ( Hawaii, Fiji, Tonga, the Solomons, Australia among others) that do practice more sustainable, harvesting methods and the two most notorious offenders ( the Philipines and Indonesia ) have areas that are "clean". The trouble is sorting this out as consumers which is not so easy AND both aqua-cultured and responsibly harvested animals tend to cost more... how many hobbyist look at price before all else?
These are complex issues that ultimately have to do with globalization, local and international politics and commerce, local issues of extreme poverty and corruption and environmental degredation from a host of sources etc.
Randy