Aggressive Glowlight Danio

Hi all,

I am reluctant to call this a problem, more of an observation of behavoir that has developed over a number of weeks.

It seems that one of my Glowlight Danio's (a group of 5 purchased back in July this year), is displaying a great deal of aggression towards his tankmates.  He is probably the largest of the 5 Glowlight Danio's (and I assume a male as the body is quite slender).  He turns quite dark in colour (almost black) and chases other Danio's around the tank for many minutes at a time.

I have even spotted him briefly chasing after Glowlight Tetra's and to a lesser extent my Vietnamese WCMM too.

From reading up around the web, I've learned that the darkening in colour and the chasing around is either due to spawning or stamping his dominance over his shoal. I'm ruling out spawning as it seems to have been going on for a good month now.

I'm curious as to whether anyone else is familiar with this, and if I should be at all concerned.  So far there does not seem to be any attempt to injure other fishes.

Thanks Sue,

I wouldn't be able to increase the numbers without getting rid of some other fish first, so your idea of giving him to someone with a bigger shoal (if needs be) is a good idea. I'll keep a watch on him.

It's now been a week since I added the extra female Glowlight Danio into the existing shoal of 4 males and 1 female.  Gradually over this week I have noticed a distinct change in aggression from the males, who seem to be less prone to chase each other all over the tank.

I'm now even starting to see the older female coming out to swim with her surrogate sister.  The shoaling of the group is happening a lot more frequently, and overall things have settled down quite a bit.