Hallo Helene,
Yes now I will be more cautious!
But it always really is difficult to learn the reason why a fish dies.
In my Spaerichtys selatanensis tank I had about ten Boraras brigittae.
And I wondered why the became fewer and fewer ….
Yesterday evening I noticed that one Borara in a good vagabond mood tried to swim up the bypass of my UV Clearer, against the water stream .. I just managed to let him slip again in the tank. And I made a filter sponge on the tube …. I think now I know where five of them had gone ….