another fish lost

The constant temperature fluctuations are too much for our little 58L tank.

Another rummy-nose tetra died this morning. Picked half-clean by the shrimp before I found him, too. I've lost four of the littlest fish recently. One thing that strikes me as curious is that it's clearly the mid-sized fish that are suffering the worst. The larger fish seem content and the smallest ones seem fine.

What gives? Do fish need more food at this time of year? I've been good at getting the dying bits of plants out as they start to go, and I've not changed anything that would impact the water chemistry. It's as maddening as it is saddening.