I recently added 5 golden (Beckford's) pencil fish to my 100 litre aquarium. It had 9 (now 8!) semi-mature corys (Julii and Elegans), and 5 peacock goby, also semi-mature, and 9 or so mature amano. They arrived as a bad mix of 4M and 1F, but there's lots of room, in a well-planted tank, with sand, driftwood and rock. On day 1 they schooled. From day 2 on a single M went full macho. He took over more than 2/3rd of the tank. I don't mind the shoulder barging, which is expected, but he was chasing all of the pencil fish, and all of the goby, until they were hiding in weeds at one end. He also started harassing amano that were bigger than him. One day 4 I noticed a Cory wasn't right. I put him in a hospital tank, but he died after 2 days, having no tail fin and very little other fins left. It happened fast and so I don't think it was fin rot. My water parameters are perfect – I ahem triple checked. This morning I found a mature amano dead. I was willing to consider the Cory was just illness, but the amano coming so soon after, well, it makes me doubtful. Yes, I do know pencil fish have small mouths. The Amano wasn't badly mauled and it may have been shock. The goby are slightly bigger than the offender, but they were cowed. I caught the offender today and removed him to a 5 litre. I'm hopeful the seller will take him back. They are generally described as community friendly. I understand the aggression between M's, but this was a M establishing a territory that took up 80% of a long tank. Building plant barriers to hide the two sides was no help. I have seen one reference to a (tank bred?) variety from Asia that may be more aggressive, possibly this was one of those. I am getting some more F to balance the remaining population. Things seem calmer in there. What especially surprised me was the aggression at the peacock goby – the colours are not similar – and the amano, which bother no-one. I wondered if anyone had similar experiences? Thanks for your thoughts, these forums are so helpful.