Not to mention... not their fault... but, why did your LFS sell them CUC.... cleaner shrimp? Crab? etc. Other than the obvious (to make money!) They were setup for failure. Also, (unfortunately) damsels are often sold to cycle tanks. Why? They are cheap, hardy (pretty hard to kill), serves the purpose for cycling and they are MEAN and aggressive! Why torture the chromis!?
ok/ now, that I had my rant about the store...
Ok/ what to do?
1st you are in the right place! Everyone on here will do there best to help you!
Ask lots of ?s and research/
In the meantime, you need good testing kits/strips supplies. Looks like you have the strips. API puts out a basic kit: here’s a pic of mine: pic 1
The ammonia test in here seems to be a little more accurate than strips/
I use a couple (including strips) to compare against each other... most pet store will also test your water for you. So, try to be sure your values are correct. Right now the most important ... 1st Ammonia (2nd/3rd) but, important.... temperature 78-80.., yes later it can go a bit warmer (reef tanks,etc) /right now go for basics (good heater/digital thermometer).... 3) specific gravity (are you checking?) You have to know this... salinity of your water 1.023-1.025 (the basics)... you use a refractometer: one of these 2-3 pic) looks like this:
Let’s start there... next, nitrite/nitrate levels. But, since you are just getting started you’ve got to watch the ammonia. I would read on nitrogen cycle/saltwater tank or YouTube video (easy to find... on here, BRStv, Marine Depot, Malevs reef .. just to name a few)
Don’t panic! Don’t buy anymore livestock, right now. If you do everything perfectly you may (still) lose some-all of your fish. Granted, we will try to help. The cycle is inevitable (part of it).., it begins when you add fish or food, then waste. It will ramp up/become cumulative over time... once your biological filtration is established it will convert ammonia to nitrite to nitrate. This has to happen. Basically, (can your fish live through it?) The ones you named are hardy. Most damsels are aggressive and territorial, they can and will kill almost anything, easily a chromis. Chromis are usually protected by being in schools (around 6 or more) But, don’t go buy more fish. The reason everyone on here will say that over and over... it’s possible you may lose your fish. But, think of your dream fish (probably $75-$300 range) or 6 chromis $60-$100 ... and, lose it all. It’s lots of work and money gone, within days. We are trying to save you from that, so you don’t have a disastrous experience. Hoping you have a lifetime hobby... that you love and enjoy... as much as we do. Also, sometimes people find out that this isn’t for them. You would rather find that out and lose $100 vs. thousands/and, sooner rather than later. Either way, it’s a learning experience! Good news: the cycling doesn’t last forever (it may seem that way!) It usually takes approximately 4 weeks.
) 
