I went to my aquarist yesterday and after a longish conversation ended up leaving with 12 Trigonostigma espeis. According to the storekeeper they do not need dark still waters like T. heteromorphas but are happy in a light tank and prefer strong current. And that has indeed seemed to be the case judging from their behavior - they're very active and love swimming against the current, then give up and go with the flow, and then swim towards the current source again. Lovely little fish. <3
It's too bad that at least for now they haven't affected the behavior of the Sids. I've seen Sids chasing them/swimming with them a couple of times, but mostly they just hang about in their favourite place in the tank, under a big branch of wood in the other end. It occurred to me today that their new behavior might also have to do with the lack of plants compared to the old arrangement - most of the plants were old and were thrown away by my sister, who rearranged the aquarium for me last Friday (I paid her a little to do it and as a poor uni student she was v. happy to have the job

Nancy - thanks for the compliment, I'll tell my sister.

Jonas - unfortunately my aquarist doesn't have his prices online and I can't remember what the species you asked cost. The T. espeis cost 4 euros/fish and regular tetras (eg. neon tetras) cost 2 euros per fish as far as I can remember. If I remember, I'll make a list of the species you asked for my next visit to his place and check the price level. Thanks for the info!