[Nina_b]

Today, I took back to the LFS my yellow lab mutzilla who has been terrorizing my tank. I am sad to see him go, but it is for the best.
My one female YL mutt is in my hospital tank to let her tailfin grow back. Her fin was basically gone, luckily it seems to be growing back. When would it be alright for her to go back in the main tank? She's less than happy on her own in there.
Second issue, how do you keep your cobalt blues nice and blue?
Don, my male, used to look like so:

Now both he is:

Would simply adding some algae wafers to their diet 'fix' it?
Currently they eat Prodak Vegetable flakes, with the occasional pea or carrot.

 

[Nina_b]

It's a 55 gallon (four feet by one foot, more or less), with one group of yellow labs (at the moment I think it's a group of 5 females - vented by LFS) and five of these guys, one of which I'm pretty sure is a male, the others haven't clearly gone either way.
It could very well be agression, as stated, the evil labzilla went back today. I tried everything else first, I moved rocks, I turned temps down, I got him a mirror (works surprisingly well - for a time). Last thing I did was get two new lab muts to try and spread agression (more females to one male seemed sensible), but he promptly killed one, and banished the other one to the filter. That was the last straw.
If it's agression, their colour should get better from here on.

 

[Nina_b]

Pretty sure he's not sub-dominant in his group. They've all gone pale and silvery, like the picture, his is just the one I have nicely documented.

 

[Nina_b]

Tank was stocked in December (cycled through November). The stock has changed, the LFS advice auratus has left and I've upped my numbers to their current ones try for harems. I will, once I've given these guys some time, probably get me another male yellow thingy.
I change 30% every other day (yes, they are growing really well They're all now over two inches. I'm so pleased with this ).
I tested yesterday, nitrates at 10, all the rest at 0. pH is a solid 8. GH and KH are fine, 16 and 20, although they're hard to read at the top end of the colour chart for me (I figure as long as they're up there, and pH is stable, it is alright - if I'm wrong, please say).

I'm glad that the consensus seems to be that it is not a feeding issue.

 

[Nina_b]

No, I don't add any buffering stuffs.
Could buffering stuff make them blue? This seems a bit out there, to me.

 

[Nina_b]

They're looking better today already. Bluer,and more fins out. Agression would now seem to be highly likely, at least for part of the problem. That's better, I might even venture to say that Don (above) is looking to become my new tank boss (but I'm breaking my own rules, wait two weeks before speculating on hierarchy after any changes).
I'm suspecting I might need to fiddle with my lighting, they look a lot bluer with the lights out, and "pale up" quite soon after the lights come on in the morning.
I have a 20 watt daylight bulb on there at the moment. Is this too bright for them?