Backyard Aquaponics
A place to discuss aquaponics

Onions might not be the best indicator for low light levels because it's not quite as obvious when they are stretching to reach the light. If it were a damping off fungus you'd lose plants very young by having a dead section just around the soil/media line and the plant would fall over. If they get larger but look spindly and thin then light is more likely to be the issue. Check out the section on what happens when plants receive too little light - http://photobiology.info/Chalker-Scott.html

Plants will look leggy and often will lean toward the light source if they aren't getting enough. Another way to describe it is that they look like they are stretching toward the light.