What's killing my gobies?

[Flashy Fins]

Last week, I noticed my most outgoing fish, a yellow clown goby, was missing. I checked the sump, overflow, and every crevice I could find to no avail. I figured he must have died, although there was no good reason for that to have happened (young fish). Now, I notice both my green banded gobies appear to be on death's door. The less-horrible looking of the two:

The other looks much worse and can't be coaxed out of the rockwork. Their breathing is extremely labored. The goby in the rocks allowed a nassarius snail to crawl over it without much reaction, causing me to think he was dead until I attempted to net him out. Now, there's no chance of me getting him out.

I quarantine all fish thoroughly. They get at least 6 weeks of observation, which includes 2 rounds at Prazi (minimum) and tank transfer method to ensure they don't have ich. I also QT all other livestock, with corals and inverts in a fish-free tank for 10 weeks minimum. If disease made its way in, it has to be one that can go undetected for long periods of time. The clown goby that went missing and these banded gobies have all been active, healthy fish in the tank for months. They ate well and played well. The banded gobies looked just fine the last time I paid special attention to them, which was only a couple of days ago.

Recent additions to the tank (all fully QT'ed):
Lubbock's wrasse
high-fin banded goby
fighting conch snail

Other fish in the tank (not recent additions):
sunburst anthias
Midas blenny

There are no crabs in the tank, only snails. There's practically no chance of a larger predator in the tank, such as a mantis shrimp, but I suppose there could be something small deep in the rock crevices, since these gobies are able to fit through the rocks in ways the other fish cannot. Still, I can't figure out what this could be. I'm sure I will lose these fish, based on how terrible they look, but I think it's important to figure out why, so it doesn't happen to any others.

What is killing my small gobies?

 

[Macdaddynick1]

I had a pink spot goby , and those are huge may be 5-6 inches across , and one day i came up to my tank and discovered my tiny coral banded shrimp is eating its top dorsal fin along with the part of its flesh , while the fish was sleeping and still alive. It was the weirdest thing I've seen . the next day the fish was dead. Perhaps the same thing is happening with your small gobies and nassarius snails, or other scavengers like big bristle worms or a fireworm .

 

[melypr1985]

I'm not sure about the cause here... it looks like a really bad case of fin rot. I would put them both in QT with Kanaplex, furan 2 and metroplex and cross my fingers. I'm sorry, I wish I could be of more help here. @Humblefish will probably pop in and know exactly what it is though, so there's the good news!

 

[Humblefish]

Based on what I see in your first pic on the dorsal fin, a bacterial infection is what's killing them. I would use the triple antibiotics Meredith advised.

Unlike parasites/worms, a bacterial disease is something you can't really keep out of your DT. Opportunistic harmful bacteria are always present in the water. What keeps them at bay is a fish's healthy immune system. What allows them to strike is a cut or open wound or even just a compromised immune system. It's very similar to our own bodies. We are always at risk of getting an infection if conditions are right.