Most light flaky fish like tilapia,haddock, pollock,etc... are pretty low in fat. But remember the fats in fish are good fats Salmon,Bluefish,Char,etc... are all high in fats but there all very healthy.
The thing you have to watch out for if your going to be eating them a lot is mercury, tilapia, pollock, and salmon are all heavily farmed and contain a lot of mercury.