General rule of thumb is minimum 60lt tank,10lts per fish although personally I think 6 in a 60lt would be pushing it but depends how well they get on.How many do you in there now? I can see one near the centre.
Curious because stocking density advice seems very mixed and it would be nice to hear from someone that knows the fish well.
Currently I have 10 in a 120lt (2m/8f) which once they go into the 180lt I will increase to 15 or so I think....some of them are over an inch snout to tail so not as tiny as some people like to think.
As with all fish that are settled in a tank you see less of the shoaling behaviour until it come to water change time