We have them in a 10 gallon tank with a filter <10 gallons is too small for adult x. laevis these frogs need at least 10 gallons for each clawed frog..> , though I'm not sure what kind it is. My dad gave it to us with the tank I replaced the charcoal today when I cleaned the tank. <Cleaning the aquarium is a good idea but the charcoal is pointless..> I have not tested the water. Can I just get a kit at the pet store? <API Freshwater Master Kit can be purchased from Amazon for about $20.> What am I looking for in terms of the condition of the water? <0ppm Ammonia 0ppm Nitrite and 20ppm Nitrate or less; Temperature in the range of 68F to 72F and a stable PH.> I did put in water conditioner today when I changed the water. <Good.>

We have been feeding the grow-a-frog pellets that came with them. <I don't know anything about this food but I know Reptomin is a good staple.>

I just noticed that his front legs look reddish underneath. I'm guessing this must be red leg. <Usually fatal, unfortunately..> His back legs don't seem to look red, though. He doesn't look swollen or anything either.
Sadly one of your frogs has died due to poor conditions caused by too small of an aquarium which led to toxic water conditions.

Now that one frog has passed I would do proper water changes on the 10 gallon tank as Louis has suggested and keep the water temperature 68F-72F. Overfeeding will foul the water, always remove excess food immediately.

Sadly if the frog does have red leg, it's usually fatal by the time you can see the symptoms.. you can go to this SITE regarding diseases and treatment.

Good luck I hope your frog recovers, proper conditions and educating yourself on what these frogs need to live and thrive is the best defense against these sort of circumstances though.