Are there adaptations in tropical fish to compensate for less oxygen in water?
Fish that live in cold water for most of the year actually have particularly strong hypoxia-tolerance adaptations, i.e. from summer heatwaves and algal blooms, compared to fish that are better oxygenated by waves and flowing rivers.
Goldfish and other fish are able to rapidly change the surface area of their gills in less than a day. 
Carp, killifish, goldfish and oscars have 2/3 times as much glycogen in their bodies than river fish i.e. rainbow trout.
There's a lot of research into the biochemistry of fish's oxygen response, i.e. gene expression changes, things like decreased expression of genes involved in the citric acid cycle.
Probably you will find most of the information on the wiki page, there are also summaries of current advances in the field.
On coral reefs, oxygen is a limiting currency for all the fish and zooplankton. The reading suggests to me that hypoxia adaptations depend mostly on heatwave events and low current waters vs rivers and coastal waves than by tropical vs pelagic and temperate regions.