I have experienced a few fish deaths last summer because of the high temperatures (up to 31'C)
I had one 280 l/h pump running and in my opinion this is way too weak to add enough Oxygen to the water.
Heavily planted tanks should have much stronger pumps or more pumps to prevent "dead spots" from forming (organic build-up).
I have introduced one extra pump yesterday which circulates 350 l/h.
Now I have 630 l/h. Almost 15 x of the tank volume per hour !!! :-) That sounds a lot but fish doesn't seem to react to it. The flow is much stronger now but the fish is swimming as usual. I would say if the current is too strong the fish would probably hide in the corner with weakest circulation.
Corys seem to be much more active in foraging for food so my guess is this tank is going to see better days from now on.
