I have 3 tanks, a 44g, a 20g and a 10g.
When I first put fish into the 44g, around 10 guppies died within the first week. I had guppies, cories, a betta and a gourami in it.
When they were dying, they looked like they were gasping for air, their gills were moving really quick.
All the guppies died, except one, I filled the 20g, and put him in there. He is healthy now.
I tested the 44g's water for ammonia, and nothing. The pH is 7.0.
Today, the betta died, the gourami hasn't been eating, and the cories are pale.
I did a 25% water change, and added more circulation.
I use the same water for all the tanks, and the two smaller tanks have had no problems.
The only difference is that in the 44g, I have sand, live plants, and driftwood. The other tanks are all fake. Could that make a difference?