Why the tetras hide: flow, light, and the empty middle
The tetras arrived in June and for the first month I barely saw them. They sat in the gap behind the filter, nine of them pressed together, and came out only for food. The tank looked empty and I started to wonder whether I had a sick shoal. They were eating, their fins were up, and the water tested clean. They were simply hiding.
Three things turned out to be wrong, and I fixed them one at a time so I could see which one mattered. The first was the light. The tank sat at full brightness for ten hours a day, and the only shade was the filter. I dropped the light to 60 percent and the photoperiod to eight hours, and within three days two or three tetras were out over the gravel in the late afternoon. That was the biggest single change.
The second was flow. The filter outlet pointed straight down the front glass, so the open middle of the tank was a current they had to fight. Tetras come from slow water. I turned the outlet to the back glass and put a piece of driftwood in the middle to break the stream, and the shoal started using the front.
The third was the floating cover. I added a handful of floating plants, and the roots hanging down gave them a ceiling. Fish with a ceiling behave as if nothing is going to drop on them from above, which is the thing they are actually afraid of.
By the end of July the shoal was out in the middle most of the day, tightening up only when I walked past quickly. I have since read that nine is a borderline number for this species and twelve would be calmer, but the tank is 60 liters and I am not going to push it. The water now: 25 °C, pH 7.3, nitrate under 10 mg/l on a 25 percent weekly change, and the tetras spawn in the moss every few weeks, which I take as the final word on whether they are happy.