If the issue is not wanting to pull excessive volume of beer/wort for testing, get a brix refractometer — they're QUITE economical (averaging $20-25) and only pull a small dropper's worth. When you use a reputable calculator for converting brix readings to factor in alcohol, it can be accurate within .005, which frankly is good enough for most home brewing endeavors.
The Brix calculator from Northern Brewer does a fine job as does Sean Terril's.
I'm not sure how good of results an aquarium hydrometer is going to produce since it has a specific purpose (measuring salinity instead of weight of sugars left in solution) and is not necessarily calibrated for measuring beverages like beer, wine, mead, etc. You may wish to do a side-by-side comparison over the course of three or four beers to test for yourself its level of accuracy. But I'm inclined to think it won't work.