Dear Helene, yes, it is very worrying indeed. The most worrying thing is the situation of the natural habitats. I often meet people who say: Well, it’s a pity what is happying there, but there are still all species present.
Yes, as far as we know all which we know are living still the present day. But we can be pretty sure that some are no longer to be found which we have never encoutered.
And the other thing that is not to be forgotten is: “Fish present” don’t not mean: “Fish healthy and natural stock without any problems; everything in order”. We have more and more reason to believe that the genetic stock of many animal species (whether of insects or amphibians or fish, etc.) are partly affected by environmental damage. Think of the huge amounts of environmental toxins and poisons that are spread in the air especially in the tropics, for isntance the countreis in south-east Asia that hope to improve their economic welfare by the huge palm-oil plantations. Much of this must get into the soil and rivers, and nobody cares.
With fish, the most likely indication is not clear outer damage of their bodies or their behaviour, but in the first line the sex ratio. The best thing I can say at this issue is: We have a few clear cases only so far.
But it is quite obvious with aquarium fish although we do not spray environmental poisons into our tanks, of course. But we must control the environmental factors we place at their disposal. Too cool or too warm, too high pH (too low is hardly possible), too high a calcium content, missing humic substances, too many germs: all could contribute to a misture with the result of an uneven sex ratio.
The difficulty is: All this works together, is a complex system. You cannot say: temperature too high, or pH too high, and so on. There is an interrelation vice versa that make things difficult. This is what science tell us. It cannot tell us for each single case: That’s it. You must assess it yourself as open minded as possible.
Nevertheless, one should not be frightened: Mostly things work out quite well. But one should know this system of environmental factors that influence the final determination of sex in fishes.