On a related matter, my Pearl Danios (was three 5.5cm adults and seven 3.5cm fry) were put through two massacres in the last three weeks, thanks to bad judgement calls by me...
The adults used to be in the Rio240 without problems, from around April to June. I then removed them to the safety of the Korall60, as my new Barilius canarensis(?) appeared to be intimidating them regularly. Having moved the Barilius and Opsarius to the 540l, I decided that it would be a good time to return the Danios to the 240l... Wrong!
One of the fry literally had its head removed within two minutes of adding the group to the tank, now thought to be inflicted by the then unknown Lionhead Cichlid parents. One of the adult female Pearls suffered a huge wound to her back and flank, but thankfully she healed well and I managed to quickly (somehow) rescue the other remaining eight and get them back to the sanctuary of the Korall.
Two weeks later, with the female having healed up superbly, I was determined to give the Pearls the freedom of the 540l. I'm always preaching about Danios needing to be kept in 4-foot tanks and here I was couping them up in a 2-footer! Given that I had seen the previous aggression levels deminish to nothing in the 540 (even the canarensis were behaving), I felt really positive about the transfer... Wrong!
Over the space of two nights, four Pearls had vanished, now definitely presumed eaten (although I'm not sure by who, I suspect the Opsarius, but perhaps it was the ~12cm SL Synodontis brichardi). In amongst the survivors was that same adult female plus a "clever" fry who had taken up sanctuary in a really tight spot under a piece of bogwood, after having almost half of its caudial fin bitten off.
In what has been an awful few weeks of fish keeping experiences for me, having lost two of my Humphead Glassfish and a second Leopard Bushfish from the group of five (no symptoms that pinpointed a cause, but I'm no longer operating the Magnum8 powerhead through fear that the 12500lph current is stressing out some of the fish in the 540l), I've learnt some lessons the hard way. It has really highlighted the need for me to properly research the conditions a fish needs as well as the "if a fish will fit in another tankmate's mouth, it is a case of when, not if" lesson.