Im sorry for your tragic loss .

A 12x12x18 really will not sustain two fire bellies for long. Recommend get a ten gallon long.

Quarantine is important. It makes sure that the frog is disease and parasite free. Just for future reference, a quarantine enclosure should include
-damp unprinted paper towels as substrate
-a shallow water dish
-a place to hide

The frogs should never be in direct sunlight. The glass is like a magnifying glass. Any old light bulb will do but a 6500k light bulb is needed for a planted tank.

Water needs to be changed more often than that even with a filter.

Gravel is an impaction risk. Especially since the crickets roam free. If the frog swallows the gravel it can become stuck in the intestinal passages and it is very dangerous. Gravel should not be used in a frog tank unless it is a few inches under water.

Mealworms have high levels of chitin which is hard for the frogs to digest. They are okay as treats but i myself would never feed them to any of my species.

Calcium+d3 powder and multivitamin powders are necessary and important. Calcium every other feeding and multivitamins once a week. Keep gut loading the crickets as well.

Petco frogs are rarely well taken care of. They may carry disease.

Not really sure what the cause of death was. Could be impaction or disease. I think there may be some pathogen floating around in the tank. Recommend placing remaining frog in QT and disinfecting entire tank with a 2% bleach solution and rinsing until bleach smell is gone then let sit in the sun for a day. Substrate and water should be thrown away. Remember to quarantine before adding nee frogs

Good luck!