Yellow Coralis Wrass

The yellow wrasse likes meaty foods like live/frozen brine, mysid srhimp. These guys will sometimes eat fan worms and are not safe will all shrimp and inverts and they will also feed on fire worms and flat worms in the tank. The recommended tank size is 30 gallons but your 20 will be fine when its a juvenile. One thing about this fish is that it likes to bury itself in the sand when its frightened. How long has this tank been setup for?

You're not going to like this. Sorry but I have to say it.

I sincerely think you've rushed into this. A SW tank should be in the 1.024-1.026 range of specific gravity. My understanding is that you've been increasing the numbers to attempt to turn a brackish 20g into a SW nano but that you had not yet reached it nor tested to be certain it had enough SW bacteria to handle the dentrification cycle. Do you have a SW test kit? If so what are your test numbers?

The coris wrasse in my opinion needs the 30g minimum, it is going to need to be protected from jumping, and it is going to need at least 2x day especially at the diminuitive size your's is at. When it gets to be an adult it may go after your crustaceans, it might not but it is a risk you'd want to consider. It is a one of the best coris for a reef tank yes, but like any other fish it shouldn't be bought before knowing about it's care.

So right now you have 3 tiny fish yes, but those fish are all going to grow. The cori is going to be close to 5", the damsel will stay under 3" but don't let the size fool you as it is a damsel even if one of the less aggressive ones. The mono gets up to 10" and my understanding is that they prefer to be in groups and don't move well as adults so it is recommended to get a very large tank and have a shoal so a loan mono isn't stressed and hiding.

It is great that you took the amano shrimps out of the equation but instead of buying new fish to add to your system how about considering the many posts from several posters in the marine forums replying to your questions and saying that the mono needs a better home? That a SW tank needs properly cycled and water parameters made suitable for livestock? We all would love for you to succeed and are obviously willing to answer questions but sometimes you might not like the answers.

What are the water parameters? Hopefully they didn't just tell you good?

You should know your :

SG (and your's is low, did you test the SG of the water the wrasse was in before acclimating it by the way?)

Ammonia

Nitrite

Nitrate

pH

and those are just bare minimum. Phosphate, Copper (especialy if you are using tap) are good for the basic tank set up as well. Start adding anything dependent on calcium and you need to test for calcium, alkalinity, magnesium, etc. The store isn't always open when you need your water tested and unless you ask for a liquid test to be run manyl run a quick dip test which is not well respected for reliability.

Does he always have his head raised up? I have 6 in my tank and all of them swim vertically unless they are swimming up or down. I don't know if it unusual for them to swim vertically but I would watch him pretty closely.

nah he won't suffocate, the yellow wrasses will bury themselves in the sand when they are scared.