Here's my detached garage fishroom (work in progress, of course):
. There's also a part 2
Make sure you pay special attention to insulation, air circulation, water and electricity. I ran water and beefed up electrical to the garage so that I could install a beefed up electrical box for an on-demand hot water heater, split unit HVAC, dehumidifier, air pumps and fans. Also had a couple of extra windows and 4 skylights installed and, of course, insulated and finished the walls (with greenboard and mold-proof paint).