Hypo-Check is probably the most handy thing to have around when working in the darkroom, or when developing film at home. The solution comes in a very small bottle, but each drop is filled with valuable information – it will tell you when your fixer is too exhausted to use. The way it does that is very simple: When you drop a drop into the fixer solution, the drop will either remain clear or turn milky. If the drop remains clear, your fixer is still good. If it turns milky, then you know you need to refresh your fixer supply. Adding a couple of drops of Hypo-Check to your fixer to test it does not harm your fixer at all.

Having good fixer is extremely important. Fixer keeps the light from damaging your film and your paper. Negatives that haven’t been fixed properly will degrade over time or appear cloudy, but don’t worry, you can always fix them again even after you’ve gone through the whole film developing process. Photo prints that aren’t fixed for long enough or with fixer that is no longer any good might look fine for a long while, but they will eventually turn brownish in color as the light damages them.

Hypo-Check is inexpensive, and aside from being available online, it is also available in any photography supply store with a darkroom section.