What the heck happened in my tank?

[waterlily]

I've had my 29 gallon saltwater tank for a year now and all the fish in it now have been in my tank for at least 6 months. I do 25% water changes every other week with Instant Ocean diluted with my well water. All of the parameters look good in this tank - temp is 78 degrees farenheit, ammonia - 0, nitrite - 0, nitrate 3-5 mg/l, pH - 8.3, specific gravity 1.023. The last time I did a water change was about 9 days ago. Now tell me why disease would pop up in this tank when I've added nothing new and not stressed the fish. My cherub angelfish died Monday out of the blue with no visable signs of disease and now I'm seeing specks on my Brazilian gramma (I assume this is marine velvet?). The other residents - a small percula clownfish, a whitespotted toby, and a cleaner shrimp - are looking and acting fine. I fear that this is going to wipe out my entire tank and if there's anything I could do to prevent this from happening in the future, I want to know.

[FMarini]

Hi:
thanks for the post and sorry for the problem w/ your tank....
So my first gut feeling is that the water change was sufficent to stress the chems of the tank, and your fish. My guess is you had subclinical levels of disease(meaning they were present, but you didn't notice them), and that this recent event was the straw which broke he camels back.
So i ask you beside excellen water quality what else about the tank?
what is your complete fish list? is it all four of these fish + somemore?
I'm wondering if you observed bickering between the fish (like the gramma?)
So any ideas from your end.
As far as "the specks" describe them, are they like cottonballs, pinpoints, dusty covering on the fish?
frank

[waterlily]

Frank, thanks for your reply. I don't have any other fish in the tank. From about early February until Monday, there were 4 fish in the tank (whitespotted toby, cherub angelfish, brazilian gramma, and percula clownfish) and one cleaner shrimp. Interesting you should note the water change as a stressor. I've done this water change thing every two weeks since I've had the tank, so why THIS water change? Maybe my well water contains fluctuating levels of certain elements and/or toxins? It goes through a treatment/softener and it tested fine with the health department. The spots are very fine, like a dusting. I have to look close to notice them. I have a microscope at work. If the gramma dies, maybe I could take a skin scraping and check it out under the scope. I don't think that would accomplish much, but at least I'd know for sure what the bug is.

I forgot to mention my equipment - Visi-Therm heater, Remora skimmer, and Emperor 280 power filter. I have crushed coral substrate and about 10 lbs of live rock which doesn't have much life other than algae and one aiptasia on it.

[waterlily]

I forgot to answer your other question - the fish were getting along beautifully. I have been patting myself on the back for months because I got a beautiful and compatible fish collection with the limited experience and small tank that I have.

[billsreef]

Any changes in your water softner, such as changing brands or types of salt. Some salts use sodium cyanide as an anti caking agent :eek1:

If you can catch the gramma, to a scraping now while the info can be usefull to save it. Typically you want to make scrapings at the base of the pectoral fins, base of the tail and along the mid section of the body. Gentle scrapings can be done without hurting the fish.