It doesn't seem like a dead fish situation. They are gradually getting more peaceful. Haven't seen any power struggle for the past 2 days. I have read that with Hillstream/Borneo suckers, they do prefer groups, but not in limited space. Now, not sure if 15 litre/4 gallons is too small for a Borneo sucker (they tend to stay around 3 to 4 cm), but could be. They are getting friendly tho.

I know that about sorority betta tanks too. My friend has 2 and one of them bullies the other at times. As I understand, 3 or more are good for avoiding ostracising behaviour. They do seem fine tho (my friends fish), in fact they are both healthy and pregnant too. Anyway...thank you for taking the time to respond to my story. I appreciate it.