give your bowl a good clean out,take out the frog leave him in a tub of tank water or some container with the old tank water in it while you scrub the tank out,refill the tank with water add your water conditioner,heat it up to right temperature and then put the frog back in..
id say take out the gravel alltogether like someone else said,in case the old food is rotting under there and causing high amonia.just make sure he has some decoration or fake plant to hide in. for now use a bare bottom tank or try buy a bag of sand,sand is only 3 dollar in home depot for like 20 pounds of play sand. you put enough in a bucket,u swish it around in some water and then drain out the water,repeat this process 3 or 4 times then put the sand into ur empty bowl,then add water slowly and gently..
about your water changes, when you change water are you just pouring cold unconditioned tap water into the tank on top of the frogs?best to fill up a bucket with 2 or 3 gallons of warmish water,try match the temperature as much as u can to 77 degrees,then add your water conditioner,then after a minute or 2 put it into your tank..
id recomend not gettin another frog for this small tank,if your other remaining frog does die too :( i would say just go to petco/petsmart and get a betta fish for now,they are hardier than the frogs and when u buy your 10 gallon u can keep the betta in there with 2 or 3 new frogs