Froggy died - what to do with the rest of the tank?

So my boyfriend is a preschool teacher and he has a 5-gallon fishtank in his clroom. He used to have a betta in it, but the kids were totally uninterested so said betta is now with me at work. He restocked the tank last week with a snail, frog, small cory, two guppies, a neon tetra, a zebra danio, and one other teeny schooling fish I don't remember the name of. Anyway, they were put in the tank (conditioned water that sat 24 hours, plants, a little shipwreck etc - it's great little tank now) I believe on Wednesday afternoon. Everyone seemed fine, all the fishies were schooling together, other critters happily moving around. Over the weekend he installed a heater and a thermostat so he could monitor the temperature, and dropped in a weekend feeder. But this morning when he went back to the clroom poor froggy was belly up. He said the tank smells to high heaven too. I'm wondering what (besides obviously removing the dead critter) is the best next course of action to preserve the health and longevity of the rest of the inhabitants?