My six-year-old has a 20g high with a 30-gallon Aquaclear filter. After some misadventures with some sick guppies last summer, she opted to keep some cories and some platy. She had one platy come home pregnant and BabyPlaty is the only baby we ever saw; I suspect the rest were eaten. That platy is about 7 months old, and the only platy still alive; the rest have died off, one at a time, over the last 6 months. Water parameters are consistently good (she and I test and change the water weekly) and tank temp is regularly 73F-75F, so I suspect they've died of old age. So now she's down to BabyPlaty and six cories. Daughter would like to restock with some new fish, would like a little more activity in her tank, and is wondering what she can/should put in with her current fish. What do you suggest?