It would be really great if someone could help me out with the problem in my tank please.
I got about 5 gourami's few weeks back and before getting them, i got the water tested in the local pet store and they said it looked pretty good. The gourami's seemed fine in the night when i fed them and they pretty much ate food well. In the morning when i woke up, i found one dead and the others still looked good so i thought it maybe a bad fish. But when i returned from office in the evening, the other five were dead too
I went to the pet store along with water to get it tested and it looked fine then too. The i thought maybe the blood fin tetra in my tank picked on them and they died due to stress, because they did this with few guppies i had before. So day before yesterday i returned back the bloodfins and the tank just had 4 pristella tetras, and again after water test i got 4 gourami's yesterday. They looked fine in the night, ate well, but again in the morning found one of them dead

but the other three looked fine and ate some pellets i gave them in the morning. But in a few hours, one of them started behaving very funny, it is rapidly breathing, opening its mouht big as if it is having difficulty in breathing....its moving to the surface breathing again again and then falling back on the bottom of the tank. I don't think it will make it and i really dont know what to do. Could someone please help me out with thsi problem as no one could tell me why this is happening
Its really sad to see this happening and that too for the second time again...i don't want to lose any more fish..please someone help me out. I had done a
water change before getting them and had also got it tested to ensure everything was fine, but still this is happening....please help !!
Do let me know in case i need to provide more information.
It is a 20 gallon tank, i have 4 pristella tetras in it from before and they seem to be doing fine, so i don't know if it is a problem with water. Please help !!!