Zebra finch death, please help! Keep 5, or get a 6th again?

[MiaCarter]

Aw, poor little guy.

His feathers look in good shape.

There's a bit of soiling around his vent, but this could simply be due to the change in muscle tone following death (immediately after death, they relax and you can have leakage; within an hour or two, rigor mortis occurs and the muscles stiffen, pushing out any remaining matter.)

Do you still have this little guy's remains?
If so, take a damp cotton ball and wipe down his belly so you can visualize the underlying skin. A thick, yellowish fatty layer on the abdomen can indicate serious illness that often leads to organ failure.
It may be hard to tell if the blood has settled and caused discoloration.

To answer your question on diet...
Absolutely.
They need more than seeds. Even a variety of high quality seeds doesn't give them the nutrients they need to thrive.
In fact, I'm not sure they would be healthy if you did seeds and a high quality multivitamin.

Here's a look at what I feed:
http://wp.me/p4JWrl-i2

You should feed some good quality seeds as a basis, but this should only comprise a portion of their total diet. I'd say that seed comprises about 30-40% of their overall diet (+/- depending on the actual bird.)

They need fresh fruits and veggies, along with protein like mealworms and eggfood.

Here's a printable list of safe and dangerous foods for finches and other birds. I have it magnet-ed onto my fridge for easy reference.
http://www.petfinchfacts.com/wp-content ... ods-V2.pdf

Just remember that birds are often hesitant to try new foods and you need to serve the unfamiliar foods in a dish or seed cup that they recognize as a source of food.
I usually give them 2 weeks of offering a food before I give up. It's rare for them to reject a food; I'd say they eat 95+ of the foods I've offered by the end of the 2 week period.

[Kuromi]

It's a little hard to see in the photos, but there are several feathers between his wings that are grown in crooked or shorter than others, which makes me think they were regrowth (possibly he was bullied or maybe just stressed and pulled them himself?).

I'm pretty positive he did soil himself during or after death, as he didn't have that wetness/yellow color around his vent normally.

Unfortunately I don't still have his remains, as I had to take him back to Petsmart to receive a refund. I suppose I should have asked about their policy on that, but I figure they probably wouldn't allow it if I didn't have his remains to bring in.

I honestly didn't realize that they needed anything more than seed if there were no breeding females present. Until I can get some egg food, I picked up a multi-vitamin to put in their water for now. I'll try to start introducing some fruits and veggies into their diet as well. Is frozen produce alright (thawed out of course)? I always used this with my sugar gliders as we don't normally keep a whole lot of fresh stuff on hand (some though, so they would get a mix of fresh/frozen).

Thank you so much for the advice and the links! I will bookmark them for easy reference.

Also, no more fighting still. Keeping my fingers crossed it stays that way.