My first post for you was rushed and narrow-minded as I was focused on a 'new tank' and so forth
Let me start over ...
First of all, I've only had a SW tank for about 2-1/2 years, and I started keeping freshwater fish 40 years ago, so take what I say with a grain of salt (or 2).
No offence to Skimmin, but large water changes (or any size for that matter) don't remove any significant amount of beneficial bacteria. The bacteria are attached to 'surfaces' and not floating around in the water column with nothing to do.
What WCs do is remove toxins/excess stuff that mother nature would normally take care of, and your skimmer/socks/reactors can't do for you. They also help replenish essential elements that are used up by your tank inhabitants.
As long as you match all the water parameters, WCs aren't harmful.
But you can't just add, say, 50% new water if it's not 'aired out'.
WC water needs to be aerated a bit before use to off-gas stuff and such. This doesn't mean you need air stones, but just a pump of some kind to turbulate it enough to cause gas exchange.
This will help bring the pH to where it should be as newly mixed water is kinda out of balance (for lack of understanding). I think New Salt Water (NSW) is lacking oxygen ... ?
Think of WCs as more of an 'airing out your house after a stuffy winter with the windows closed' kinda thing.
Or better yet, that swimming pool that all the kids have been peeing in.
Would you feel better jumping in after someone has changed a bunch of the water ?
It's good to do. You just have to decide how much and the frequency that matches your budget/schedule.
Don't go nuts and do 50% every week. Read up and decide for yourself.
So your tank is only a couple months old.
1-How did you start it ? Describe your build as far as adding water (RO or Tap) and how you mixed the salt.
2-Do you have sand ?
3-Tank temp ?
4-Salinity ?
5-Did you test for NH3, NO2 and NO3 after the LR was added ?
6-Did you keep testing until the NO3 was near zero ?
7-How long did you have the tank running for with the new Live Rock before you added any critters ?
8-What critters did you add at what times ? As in how many days/weeks apart ? For instance, snails @ week 4, then some crabs @ week X, then the Tang @ week X, then the Clowns @ week X ....
You said the pellet food was only eaten by the Clowns and you got it from your LFS
9-Does it smell fishy (it should) ? If it smells moldy, or has no smell, return/toss it.
10-How big were the Clowns ?
I've heard of smaller fish choking on food and suffocating.It's probably not the case with you losing 3 at once, but not unlikely if they were small enough.
11-How long were the Clowns in the tank for before they died ?
12-How did you acclimate and introduce them to your Display Tank ?
13-How long after your WC was it before the Clowns started their death rolls ?
As Borderjumper asked;
14-Did all the Clowns come from the same source at the same time ?
15-What food did you feed the Clowns ? (describe the pellets, and anything before the pellets)
16-What do you feed the Tang ?
17-Do you have an ATO system ?
18-If not, do you check your salinity frequently ?
As I eluded to in my last post, sometimes critters are captured in ways that will cause them to expire and, sadly, quite quickly. Sometimes the capture methods will affect their systems in ways that they will live years, but nowhere near their life expectancies. And the way they are shipped etc comes into play too.
It's a crap shoot when you buy fish ...
Try your best to answer my 18 questions (some in detail) and we'll all see if we can help you out
Again, sorry I didn't help you out much to start with and this is my way of making it up to you
Canreef is a great resource you can count on and my initial contact with you is a poor example
Good luck !!