Fish keep dying in an established tank

[zeromus-x]

Hey guys... didn't know where to post this, so I figured this was my best bet.
We've had a 75-gallon tank set up here at work for almost four years now. Before that, it was up and running for probably ten years. Nice tank, drilled, we're running another tank with filtration under it (bio-balls for right now), pumps, powerheads, the works -- a pretty nice tank. It's always been fish-only, with lava rock and crushed coral, but we decided to convert it to live sand with live rock.
Over the course of 2-3 weeks, we slowly removed some crushed coral and replaced it with sand, etc. We had two damsels, a black angelfish of some sort (I didn't buy it, not a clue what it was, might not have even been an angel), and a chocolate chip star in there. I moved the angel to my home tank and we left the damsels and starfish in the tank during the move, trying to stir stuff up as little as possible. Eventually we finished the switch. After a few days, I bought two large chunks of live rock and put 'em in the tank. We did a 10% water change after every substrate switch, and after placing the live rock in the tank.
After a few days, I noticed the damsels weren't swimming around anymore. Ruh-oh. Finally found 'em, both dead. CC starfish upside down and dead. No good. Cleaned 'em out and did a larger water change (~20%). Let the tank sit for a few weeks. Brought my test kit from home -- tested nitrates, nitrites, ammonia, PH, and hardness. With the exception of some pretty hard water, everything looks perfect at this point. Decided it was time to move the angel back into the tank, and put a yellowtail damsel in there too. They both acted fine. Next day at work I didn't really see them. Next day, dead.
I just did another water test with that kit and nitrates/nitrites are literally at 0ppm, ammonia is at zero, pH is exactly where it needs to be, water salinity is right in the middle of the meter. What else should I be checking for here? It's not making any sense to me. There are about twenty-five snails in there that are living just fine, it's a clean tank, nothing has spilled into it, all the readings I've got are coming up perfect.

 

[trigger11]

On the surface your water paramaters seem ok. I would have them double checked by a LFS just to be on the safe side.
It is sounding to me like the problem is either the live rock, or the live sand? What kind of sand did you get? Arrogonite (sp) or some different kind of sand? When I was first reading your description I was thinking the problem may have been that the live rock was not cured. Which would have cause a new cycle with a big ammonia spike. That could kill the fish pretty quickly. But from the sounds of things you let things sit a while before adding other fish and tested the paramaters in between too. So the LR not being cured doesnt sound like the entire culprit.
How did you remove the CC out of the tank? If you used a bucket or a ladlelike thing maybe it had some bad chemicals on it?
Thats about all I can think of.

 

[dawman]

What exactly is your water levels ? Something that looks good to one is bad to another . Also , how old is your test kit ?

 

[promisetbg]

Are you using a hydrometer to test SG. ? If so, get a refractometer & test your SG. accurately. Hydrometers are innacurate generally, and get worse with time. I had a problem in the second year because I was using a hydrometer, old fish were used to it...but new fish & inverts would all die. Sg. was 1.030 when tested with refract. Another thought...how much LR did you add? You said two pieces, it does'nt sound like enough. Are you running a protein skimmer? Are you using RO/DI water? I would do a water change & run some carbon just of the off chance that something foreign did get into the tank. Lastly, how did the fish act before they died? Any outward signs or did they just disappear? Any chance of a mantis in your new LR?

 

[iowafish]

Another thought... how deep was the crushed coral bed before you removed it? It's possible there was something stagnant lying in wait (chemical-wise) that got released into your system when you disturbed the bed to remove it. ...perhaps this 'something' isn't a compound you're currently testing for? Just another $.02 to add to the pot....
I also 'second' what others have already said: switch to a refractometer (if you don't already have one) and having the LFS check your water chemistry are good suggestions as well.
KH

 

[saltn00b]

two things strike as possibilities when dealing with old tanks.
1) you moved the substrate that has been established for ten years +? there had to be a substantial amount of anearobic bacteria released into the water column that could kill any and every thing.
2) phosphates, classic old tank syndrome, that you are not testing for. although usually harmless for fish, it could have built up over this period of time to dangerous high levels. although, it should have been immediately absorbed by the new LS/ LR, and the fish were supposedly fine before hand.