Puzzled as to what is killing fish?

So this is more a beginner question, but I'm somewhat puzzled to what the problem is.

I have a 30G tank that's been up for about 2 1/2 months, cycled for a month prior to that. After cycling, I added 2 occelaris clowns and a clean up crew, all was fine. About 2-3 weeks later, added a Flame Angel, then a week or two later a 6-line wrasse.

Within a week of adding the wrasse, the flame angel suddenly died. He was healthy, swimming and eating normally, then one day looked like he was having trouble breathing, then saw him laying on his side in the sand. Within an hour, he was dead.

About a week later, I added a yellow-tale damsel. Within a week, that damsel appeared to do the same as the flame angel. Color started turning pale, he began to have trouble breathing then was dead within hours. This was about a week ago.

Yesterday I noticed both clowns were just on one side of the tank, kinda swimming in place like on a treadmill all day. Both looked healthy, swimming fine and eating still, just kinda stayed in one place otherwise. They're usually really active and all over the tank, so I thought it was a little unusual. This morning I woke up to find one of them fine and swimming around like normal and the other dead.

I'm kind of puzzled as to what I'm doing wrong or what could be killing the fish. All water parameters have stayed steady and fine (listed below). At most, ammonia spiked to 0.25 once, nitrite the same. Other than that, they've stayed normal at 0.

Could this be overstocking or adding livestock too fast? Or could it be some sort of disease in the tank? I thought it may have been some sort of aggression or stress from the 6-line, as it all seemed to occur after he was added, but I haven't really seen anything from him. When adding everything, I have been using the drip-acclimation over the period of about an hour. There are 25-30lbs of live rock, which seems to be fine as no real spikes in water parameters have occurred.

this is my first saltwater tank, so I'm kind of at a loss of what's happening. Any help would be appreciated.

Water parameters
Ammonia: 0.00pm (highest spike 0.25ppm once for less than a day)
Nitrite: 0.00pm (highest spike 0.25ppm once for less than a day)
Nitrate: 10-20ppm (highest spike 40ppm)
pH: 82
Temp :78-80
Salinity: 1.023

 

Did you see any parasite signs on your fish?
do you have enough surface agitation on the tank?
also, have you gotten all your fish from the same vendor? Sometimes the fish are already sick from the start and just end up dying on our tanks after you buy them. Make sure you always check the fish are healthy by asking the LFS to feed them before you buy them. Check for obvious parasites, odd swimming, ripped fins etc...

 

They have all come from two different vendors, and none have shown any signs of parasites or anything. No ripped fins, no white spots or anything. I did think that with the flame angel, as I have heard of some cases of them having somewhat of a bad mortality rate because of the stresses from and methods of capture. But when the others died as well, I began to think it may be something else.

 

It's probably a combination of stress from over crowding, new tank syndrome and disease.

A pair of clowns in a 30 is plenty.

It can take up to 6 months for some tanks to completely cycle depending on type and amount of sand, rock. etc.. If you are detecting ammonia your tank isn't cycled.

There isn't much you can do at this point except wait it out.

Disease pathogens will always be present in your tank. Keeping your fish healthy so they can fight is off is illness is the key to success.