L183 dead this morning :(

I have a group of L183 in their own tank. There is 1 adult male 4 inches. And 5 (now 4) 2 inch+ fry in the tank. It is a 20L stacked with driftwood, barebottom, with a penguin 350 HoB with a sponge intake on it.

I checked the water after I found the little one dead.
I used a freshwater master test kit.

Ammonia 0ppm
Nitrite 0ppm
Nitrate 20ppm
PH 6.8-7

I don't understand. I feel like I did everything necessary for them. Monday was my last water change. 25% (5 gallon bucket). The water was half R/O half tap. Which gave the water a PH of about 6.8. Equal to the tank.

I bought a TDS meter. And the TDS read about 150ppm after the water change. My tap comes out around 200ppm. The RO/tap mixture I used for the water change was about 100 TDS.

And again this was on Monday.
Idk.

Thoughts?

i'm hoping you mean its a 20G tank not a 20L tank cos if it is only 20 ltrs then there is your problem and no need to investigate any more.

If they are in a suitable tank then Two things come to mind, water temperature and natural selection. both equally as capable of killing a fish.

Sorry. 20 gallon long tank. Another was dead when I went home.

I keep the temperature around 80.

Do you really think they could of killed each other? Or I guess would have killed each other. Obviously capable.