Look for white wormy things, little bugs, stuff like that I guess. I've never had that problem so I've never actually seen it myself just going by things I've read on the forums. Frogs never close there eyes to sleep, atleast none of mine do. They do tend to dialate them a bit and their pupils get real small. I'd change the substrate once a month as long as you spot clean and pick out any ****. Since you have a big 50 gallon tank, you could probably do it less than that if you be sure to spot check and stir the soil while adding some fresh spring water while stirring once a week, that's usually what I do. I have a 20 gallon long and change it around once a month.