So, I talked to the local fish store and described all that happened. The catfish that died MAY have been sick. However, the guppy died because of.....*drum roll*
Overfeeding!

I am a bad fishy mom! Come to find out these algae wafers are NOT good for the fish because they reabsorb water...get huge...float off...and cause the ammonia to rise. And while rearranging my tank this morning I found a gazillion off-white fuzz balls floating around. So, I got an air stone (like I needed...but I had to buy another extension cord too...ugh) and the air pump has gotten the watter really moving and helping the filter clean up the crud. Was advised to do a 30% water change tonight then go back to the fish store with a water sample this weekend. And hopefully then I can get another guppy to replace Leo.