Just to mention again, but it's obvious, the Ph was no issue at all. Mine is either 7.4 or 7.6(stupid API test) and corys are fine.
I think it's better if you consider adding a second filter for a start if all you have is a 1000L/H on a 300L tank. In the long run you would need better filtration than that and a filter will also add to the flow. Then one small powerhead additionally which shouldn't disturb your sand depending on how and where you point it.
The surface movement was really a suffocating one so I agree with all you need to address it urgentlly. It's not just the fish that need oxygen in a tank, almost all other important processes need it.
I think it's better if you consider adding a second filter for a start if all you have is a 1000L/H on a 300L tank. In the long run you would need better filtration than that and a filter will also add to the flow. Then one small powerhead additionally which shouldn't disturb your sand depending on how and where you point it.
The surface movement was really a suffocating one so I agree with all you need to address it urgentlly. It's not just the fish that need oxygen in a tank, almost all other important processes need it.