Urgent Please Help

Fish begin breathing rapidly, hang out at the bottom of the tank for a couple of days and then die. It started with my royal gramma and then my starry blenny. I only have two fish left! Nems, coral and invert all seem to be doing fine.
Only recent changes in the tank are a maxima clam and I bought and a new bucket of salt(IO).
Started right after I got a small outbreak of diatoms on my sand bed about a week ago. Not sure if it could be related, just want to cover all the bases.
I tested everything I could with my kit and readings are good-ammonia, ph, nitrates. Temp is at 78 and salinity is good. LFS said they can't currently test water because they are out of tests(offered $$$).
Tank is 29 gallons with about 35lbs of healthy liverock and has been running over a year. I have two powerheads and a HOB filter that I run chemipure in.
I plan on testing for stray voltage tonight.
Any thoughts or advice would be appreciated.

Edit-Forgot to mention that I change water once a week and use a 4 stage RODI system

 

Sounds like disease.
They have plenty of O2. I doubt stray voltage but rule it out anyway.
The maxima could have brought in a pathogen or some stressor caused an already present pathogen to rear it's head.
In a 29 gallon tank 2 fish and inverts is actually ideal in my opinion.

How much water do you change and do you allow it to sit overnight prior to use?

 

I mix and change out 3 gallons every week. I let it sit for an hour or so and bring the temp up to match the tank temp. I then slowly add it to the tank over about 15 minutes time.

 

It may just be me but I find that IO is not the easiest salt to mix. Typically I mix it the day before and let it sit over night. If you want to mix and use right away other salts like IO Reefcrystals mixes faster.

 

How long should I wait to add more fish if they all die?

 

Am I correct in thinking that the advanced test kits are more for coral than fish? If my basic kits results are fine it's probably not my water quality?

 

sounds like a case of marine velvet or brooklynella. I had that and it wiped out my tank within 72 hours. I left my tank fishless for a little over a month to let it run its cycle and then start introducing fish again at 1 per week. The one thing that i changed and hasn't had me have an outbreak since (knock on wood) is quarantining all fish before adding them.

 

Couple things here...As for fish, dont add anything till you find the issue. As stated sounds like it could be disease. Could you please post your parameters instead of stating that they are good? Salt could be a problem if not mixed completely.

 

Yes the big 3 Ca/Mag/Alk are more important for corals.

 

What test kit do you use? I've had false readings from API.