Although the key descriptions of gobies and darters are similar, darters are much more common in fresh water, and gobies are much more common in brackish or salty water. If you can look at them from below, they have very different pelvic fins:
Although the key descriptions of gobies and darters are similar, darters are much more common in fresh water, and gobies are much more common in brackish or salty water. If you can look at them from below, they have very different pelvic fins: