First set back

After reading article you linked about the oxygen, i highly doubt that was it. Skimmer was running 24x7 and it's only a 55g tank, that should provide plenty of oxygen.

Wonder if perhaps Hawkfish got em? They did not look like they were beat up tho.

What could be the possible toxic things for them to eat in the tank?

 

I can't know what is going on, but I'lll offer some guesses...

1. I don't think magnesium is the culprit, given that you're not dosing it. I think that is probably just a consequence of a good specific gravity and not in the range to be a threat to fish.

2. There are too many tanks significantly more oxygen starved than yours. I don't think oxygen is the problem. The skimmer, as you say, probably eliminates this as the risk.

3. Stuff ends up in our tanks all the time without us realising it. I've had fish killed because I ran one of those oil based scented thingies too close to the tank. I've had stray cleaning sprays cause problems. I know of people killing livestock because they didn't properly wash their hands. All of these could cause the problem you describe.

4. Something else got them and it appears to be something difficult to test. My recommendation is to throw a polyfilter in there. It will not only pull out some stuff you may not be able to identify, depending on its color change, it may help identify the problem. Along with carbon and a skimmer, the three do a relatively good job of pulling out the stuff you have a hard time finding.

It sucks to lose fish, but a new tank is just not as effective at taking the knocks as an older one. Watch carefully and you may spot the problem. In the interem, set up defenses to strip out toxins.

 

Great suggestion on the polyfilter. I had no idea those existed. I'll run that in my skimmer return instead of the carbon+phosguard sock.

I'll admit I do not always wash hands before putting them into the tank, when I do i do not use soap and just rinse them off. I definitely do wash them afterwards.

I'll make sure to wash them every time before putting them into the tank from now on.

 

Titanium Grounding Probe installed. Voltage is now 0. Can't hurt to have it right?

Polyfilter ordered. Let's see if it turns some weird color. I've also oredred SpectraPure RODI unit. I'd rather know for sure that RODI i am using is perfect.

Will watch the tank for a few days, see how the fish react. If everything seems normal, will definitly get another rock beauty.


Thank you everyone for taking your time to reply and advice. 3reef rocks.

 

Good start. You can have my Rock Beauty, he nips at most of my LPS.

 

Quick update.

The other two fish seem to be just fine. Water params are still perfect. Inverts and coral are striving. Clener shrimp molted last night and is running around his usual self. Emerald crab is fine too.

I did a 20% water change just to be sure with freshly mixed RODI and brought down salinity/gravity to 1.023.

Starting to really lean towards the murdering arc eye hawkish rampage theory.

 

Wow. Just lot another fish. Hawkfish is dead. Once again everything seemed fine in the tank, water params are still ok. All inverts are ok and coral is fine.

Last night before bed I checked on all the fish and they were fine, swimming around all happy. Hakwfish was perched on his rock and seemed colorful and healthy.

This morning I found him on the substrate, barely alive. What's weird is that his forehead (area above the eyes) was very discolored this morning. Looked grey/blue. When he tried to swim he looked extremely weak and landed upside down on the substrate. 10 minutes later he was dead!

What the heck is happening to my fish?

Params are still in check just like in the first post. Grounding probe was installed. Surface pump + 2 echotechs + skimmer + power filter are running 24x7.

I am at loss.


P.S. The only remaining fish, green chromis, looks totally fine, happy and healthy. All the inverts including the shrimp and crab are fine and do not show any sign of stress.

 

Also, i've studied the hawkfish and besides discoloration on his forehead and overall pale colors, i did not notice any signs of ich or any other obvious issues.

 

Read through the thread, at first I was going to chalk it up to new tank syndrome. Even if the levels for nitrite and ammonia are 0. But now that the hawk fish has died, with no outwardly signs of disease, and you have pretty much throughly ruled everything else out, the fact that they are dying at night makes my wonder if

1. You have a predator present, do have a serpent star?
2. O2 levels fall at night, try to get an ORP reading if you know anyone with a probe or have access to one.

Are you still powering down the ecotech at night?

 

I've changed ecotech to from 25% night mode to 50% night mode when this started happening (a week ago). I've also turned surface powerhead back on. That means we have 700gph surface agitator + 300gph power filter + 300gph skimmer + 2x600 (50% of 1200 per) ecotech ... thats still a lot of flow with plenty of oxygen exchange.

Nonetheless, I'll see if i can get an oxygen probe somewhere.

I have not seen any predators or stars. Just emerald crab, cleaner shrimp and a few very small hermit crabs + tons of various snails.

Could it be coral killing my fish? Zoas? Finger Leather?