ok, so higher temps make the ammonia more toxic, lets assume this was the problem. maybe there was nitrites too. that is why i mentioned the salt. salt reduces the nitrite spike in the fishes body. its a commonly used thing for that. the fish can get what they call brown blood disease.
many fish that are in ammonia water will spin around and swim weird. ive seen australian bass do it and changed the water and they stop it within five minutes. the other thing is when australian bass are tiny fry, and the water has ammonia or too much other nutrient you will see the swim bladder is enlarged on those affected in a tank while others are fine in another rearing tank.
then, the ones in the polluted water will float to the top with a big swim bladder and then die over time.
so maybe, this is the same thing that can happen to arows.
with goldfish, with nitrite spikes, you can see the effects in the tails.
the tail will end up with bubbles of gas all through it.
when a scuba diver has a build up of nitrogen, which is a form of ammonia, the diver will have bubbles of nitrogen form in his blood too. that can then then dislodge and go to the brain and kill them.
so the thing to do is remove the nitrogen from being in the water so that some of it can come out of the fish and go back into the water across the gills.
with scuba divers, they will put them in a decompression chamber.
another thing they can do if divers are going to be deep for long periods and be at risk, is to give them special air bottles without so much nitrogen in the air tank. that way it cant build up as fast cause they do not have it in their intake.
so the fixes are, lower the ammonia levels via water changes or better filtration or by stealing bio media from another tank.. lower the temp because the ammonia is less toxic of a form with lower temps, and add some salt to reduce the nitrite spike.