water change? stress?

[DJUsul]

HI!
I started my new reef tank about 1.5 months ago. Water has been cycling with live sand/rock after two weeks of live rock cycling. EVERYTHING seems alive and well. Bugs all over, "barnacle-like" things fanning the water. I supplement the tank every few days with calcium supplement and usually have to add water to my topless 55gal once a week. All my test are on the money, and my nitrates stay around 6 according to may Hagen test kit.
I thought it was ready for a test fish. My baby yellowtail died within 3 days. The bugs were happy to feed on him, but i was sad.
Any suggestions?
Since i don't do a water change and all my reading are fine, could there be a problem there? Is it my nitrate level? Could my fish have simply been stressed?

Any suggestions would be greatly appreciated!

[Tbor]

You say all your tests are fine, could you post the numbers? The nitrate level should not be a problem for fish, although it may be for inverts. Did you feed the tank at all while you were cycling it? A cocktail shrimp, some flake, or anything? Depending on how much LR/LS you have and how well cured the rock was before you obtained it, you may not have a very strong population of de-nitrifying bacteria present. So your tank may appear to be cycled, but can go through smaller cycles with ammonia and nitrite spikes every time you increase the bio-load (add a fish).
Also, you mentioned that you have not changed any water, just topped off. Infrequent water changes are fine for an established tank. However, in a new tank with cycling live rock even if your "levels are fine" there has been some degree of dieoff (sponges, etc.) from the rock and sand. This can release quite a large quantity of toxins into your tank, toxins that none of us test for. A water change before you add another fish can only help.
All in all we need some more specifics on your tank contents and water parameters.

[DJUsul]

my specs:
ph: 8.25
Alk: 7.5
nitrite: 0
Ammonia: 0
Nitrate: 5.1
Salinity: 1.022/30ppt

The company i bought the live rock from instructed me to NOT put the sand down at first. "Cycle the rock until the sponge dissipates. Vacuum it up, and lay the sand down. Within a few weeks remove the dead sponge." I've done this, and all seemed fine. They never mentioned the "shrimp" thing, which i've read about here.
I'll change the water some.

[beaslbob]

It seems to me that a tank so new should not have to have calcium added.

I suspect that the system is still inmature.

I would recommend you:

1) stop buffering the calcium
2) make sure you have a thriving plant life established. Add macros if you do not have a good growth of hair algae or if you have added cleaner crews to contol the algae.
3) not do a water change.
4) wait a week
5) then try a single male molly for cycleing
6) don't add food for the molly for a week.

By establishing plant life and waiting you allow the system to become complete and more stable. the molly is just an inexpensive and very hardy fish to start with. Not feeding that fish does not mean the fish will starve. It will still have plenty of food by cleaning the rocks and tank. By not adding food you reduce the bioload and prevent rotting food from fouling the tank during the first few days. During that time the fish may just ignore any food you add anyway.

Finally and most importantly, the thriving plant life will vastly reduce or eliminate any ammonia spike which the new fish may cause.

[Tbor]

Only problem with adding a lot of macros to a display tank is that they can be quite hard to eradicate in the future. Not a problem if you're FO, but they will grow over, under, around and thru corals.
Plants will be great for soaking up nutrients, but they won't soak up toxins. Do you have a skimmer? Is it any good? Are you running carbon?
As far as the people you got the rock from, if you had dying sponges on it, they can take a LOT longer than a few weeks to die off. I don't see how the order of operations with the sand and rock really matters.
Adding a molly has the same effect as adding a cocktail shrimp. Except in one case you most likely kill a fish. The molly will pollute the tank whether you feed it or not, it's still eating something and excreting wastes.

[Cearbhaill]

I would also try to get into the habit of topping off more frequently. That's a lot of fluctuation.

[mandktod]

Agree with cearbhaill, was the 1.022 before top off or after, my 55 goes through a gallon a day most days so a week of that could raise your SG really high, top off really should be done each day or no more than every other day, I have a mark on my tank that tells me it needs to be topped off, I can tell at a glance from across the room it is time. Could have been many things but what concerns me is devil fish can put up with about anything and your died in 3 days, there is a problem, either your tank has not cycled yet, or your SG fluctuations were too much for it, temp fluctuates too much.

Start a process of elimination and figure out where the problem lies, because it is there and only you can see your tank.