Black caps are nice, but I prefer the orchid dottyback myself. They're one of the most tolerable and we'll behaved dotties/ basslets out there. I also have a small harem of blue/green chromis. I never thought I'd enjoy them as much as I do now. I love them because they stay out in the open, cruising around your tank, they school together so nicely, they're colors pop against a black background(I've seen them in tanks with blue backgrounds and the same can be said) and they're very peaceful.
Other fish I might reccomend for a 150 depending on what you're looking for are:
Other tangs- if you're really into tangs you could get away with adding 2 more without crowding things. Don't get anything that gets too large tho (hippos, naso, clown or sohal) and get them around the same size as your yellow. Yellow tangs are more tolerant than some of the other tangs out there so you should be just fine. I love the powder blue, but a word of caution: they can get to be quite nasty, so b sure to add them last if u want one. Kole tangs are pretty, powder brown, white cheek, and convicts are all great choices.
In my mind the sand sifting gobies are over rated. Sure they sift your sand, but they're doing it looking for the critters that make for a healthy sand bed. They make your water column cloudy for the most part, and they don't always do so well in captivity. Fighting conch snails are my go to when it comes to cleaning my sand bed. No they don't fight. They're peaceful little critters, they leave you're sandbed very clean without stirring it up into your water column, and they don't eat the things that make your sandbed healthy in the first place.
Dwarf angels- these guys I've always liked, coral beauty, potter's, bicolor, and flame are all good choices to name a few. Only one angel per tank is reccomended to avoid agression issues, I have seen people successfully keep multiples in larger tanks such as yours and that's a gamble I'm willing to take, but that decision is ultimately up to u. Again add them at the same time to better your odds of success. Have a plan B if it's not working out so you can get one out of there and in a new home before it turns fatal.
Green mandarins are awesome, if u have a fuge going with pods they will be just be fine in you're tank, look for one that accepts frozen foods. I have mine in a 45gallon which some people would say is way too small for a mandarin for long term survivability, but mine accepts frozen mysis, my fuge is teaming with pods and he's as happy as can be.
Firefish- beautiful, peaceful, hardy. Love them.
Blennies- I too really enjoy the Blennies, almost picked one up but the orchid dottyback was too beautiful to pass up. Lol anyway, canary, midas, and starry blennies are all great.
Foxface rabbit fish are another one of my favorites, the magnificent foxface is one of my all time favorite fish. The white body paired with red and orange tipped fins are just absolutely stunning to see in person.
If this was my tank the stocking list would look something like this:
2 ocellaris clowns
3 banggai cardinals
1 yellow tang
5 blue/green chromis
1 orchid dottyback
1 yellow wrasse
1 magnificent foxface
1 powder blue tang
1 flame angel
1 bicolor angel
1 green mandarin
1 red linkia starfish
Clean up crew:
10 cerith snails
6 nassarius snails
5 astrea snails
5 trochus snails
3 turbo snails
3 fighting conch snails
1 scarlet skunk shrimp
1 flame shrimp
2 serpent sea stars
10 scarlet reef hermits
3 blue leg hermits
3 red leg hermits
3 zebra hermits
2 emerald crabs
Variety is key to a successful CUC.
