What caused my saltwater fish to die?

Question

My fish died overnight, but my 3 corals, starfish, crabs and snails are still alive. My nitrates were high so I've done 3 water changes (20%) in about a weeks time. The fish weren't stressed and looked like they died while swimming. please help

The tank is established with everything (ph, ammonia, salt concentration...) perfect except for the nitrates. We had hardy damsels and a lunar wrasse, yellow tangs, coral beauty, hawkfish, keyhole beauty and a pseudochromis. I can't understand how these fish could die, while the coral can still live??? They literally looked like they died while swimming, their fins were flat (not like they look when stressed) and the wrasse was even curved around (like he was turning around in the water). The only thing I could think of was electrocution (which should have affected the corals too) or a bad batch of brine shrimp (I just bought it a week ago so it wasn't old, but only fed it to the fish the last 2 days).

The tank is 180 gallons and the last fish were added months ago, while the corals were just added last wed. There were no outgages of power. What do you think about the brine shrimp possibly being tainted?

Answer 1

Couple questions - what size is the tank, and have you added anything recently (fish, inverts, decor)? Any power outages or reason that the electric might have been off to filters? I'll check back in a while to see if you've posted an edit with answers.

ADDITION: I suppose it could be a possibility, but I'd think it would be unlikely. Unless you rinse the food really well, anything that would be on/in the shrimp would be in the tank and should affect your corals as well - unless they are just slower to show the effects. Did you notice anything odd about the way the brine shrimp looked? If they got held up in shipping, or the store didn't get them put away quickly enough, they might have thawed and started to decompose, then were refrozen.

Since the corals were the last things added, you didn't get any fire/chili corals that might have stung your fish? Although to get all in one night should be pretty unlikely too. Or you might have unknowingly introduced a small mantis shrimp that growing up - but this should be evident if they were attacked.

I was thinking if there was a power outage, especially overnight, the fish would have been at the bottom of the tank where the oxygen would be depleted more quickly, yet if your corals were closer to the surface, they might have gotten enough O2 exchange to survive. I'll think on this a while and see if I can come up with any other possibilities. Just about anything I've come up with should have either affected your corals first, or at the same time.

Since you brought up the possibility of electrocution, have you checked the wiring on all the equipment that goes into the tank - especially anything near the waterline (unplug before checking!). The way you described the curved wrasse would seem to indicate this is a possibility at least - I've done stream surveys using electroshocking equipment, and if a fish runs into one of the electrodes it can cause this effect. Did you top off the tank yesterday which would have raised the water level? But I wouldn't know why the corals didn't get shocked as well. Are you positive they're okay? Anyone in where you live besides you that might have knocked something electrical into the tank and got it back out?

You didn't happen notice any unusual activity or marking/coloration on the part of the fish the day before this happened? Gasping for air at the surface, more/less active than usual, thickening of the slime coat?

Answer 2

I will venture to say it wasn't the PH for the main reason, currents and waves change the ph in salt water on a dime. One moment it is 8 the next 8.7.

When you say high nitrates (not nitrites?) what do you mean by high? most salt fish can withstand nitrates over 120 so that isn't high and your corals and starfish would have died first, so it wasn't the nitrates.

How old is your tank? and what kind of fish were they that died? There could be a few reasons they passed away and if that is the case, your starfish and coral will be next.

Your marine tank if immature (bacteria) will not sustain less hearty fish.

Also, when adding new fish to a salt tank should only be 1 fish every 2 to 4 weeks and only 1 inch of fish for every 5 usgallons in the first 6 months of cycling a tank. (regardless of what filtration you have.) this allows the biofilter time to readjust to the new inhabitants.

You have to go slow for the first 6 months to a year and allow for your filter to catch up with newly added fish. Snails, plants, shrimp corals etc. all count as a fish.

After the first year you can double that amount.

The bacteria in your salt tank can be easily described as the thin air on a high mountain. Although humans can breath it, additional oxygen is required for Health! even though Mt. goats can live there very little else can. This is the same in your tank. The bacteria needed to sustain certain forms of life needs to be pleantyfull not just enough to eek by.

Feel free to email me and I will try to help further.

ADDED:

I have raised salt fish for 30 years and never got a bad batch of brine shrimp. That isn't to say it isn't possiable, however even the residue from bad shrimp (which more than not the fish wouldn't eat) would have also affected the coral. Also, coral doesn't die like fish. They take a little longer. Also, fish really cannot get electrocuted while in the water. So that too is out (or a far stretch).

here are a few remote chances of what happened
Cyanide Poisoning
Almost exclusively occurs in fish or corals captured or harvested in certain areas of the South Pacific. Do you know where your corals came from? Sometimes this can tank three weeks with no symptoms until the fish have died.

Oxygen Deprivation
Normally caused by lack of vertical water movement in a tank.

Answer 3

marine fish are just VERY sensitive. They could die to the slightest pH temp. change...

Answer 4

Did you chech PH in your water?

Answer 5

Did the water have any salt...:>)