A little guidance please

Hello All,

First the specs:
90gal
T5HO lights
Lunar LED's with shimmer
Wet/Dry Sump with Protein Skimmer
Hydor Koralia 1400
75-80 Lbs Live Rock- Approx
3-4" Live Sand bed

Ammonia- 0
Nitrites- 0
Nitrates- 0
pH- 8.3
SG- 1.025

Current Livestock
Powder Brown Tang- small
Blue Tang-small/medium
Pair Ocellaris Clownfish- small
Gold Head Sleeper Goby- large
Hoeven's Wrasse- medium large
Sixline Wrasse- medium
Red Head Solon Fairy Wrasse- medium
Flame Angel- small
4-5 Hermit Crabs
4-5 Cerith Snails
4 Turbo Snails
Coral Banded Shrimp- large

Coral
Lemon Tree- very small
Green Fluorescent Mushroom- 1 or 2 on a small rock

This is my 1st saltwater tank. I have been doing fresh and brackish water tanks for 10+ years. The saltwater tank was already established(Running for 5 years) and believe it or not was moved with over 50% of the original water, giving me a great start. The tank has been up and running for 8+ months. I have been enlisting help from friends and family that have been running saltwater tanks for years.

My issue is that I really like the Tang family and every time I have added them to my tank they just don't make it long. I am well aware that they are somewhat ick magnets, but every last one I have added to my tank dies(ick or otherwise)? I'm looking for some direction to help keep the Tangs and allow them to thrive. Also, direction as to were to go with the tank... additional fish(not many more)... coral (easy, not looking for a reef, just diversity)... etc. The last thing is that I order 90% of everything online cause there is just not a good shop close to me.

Thanks for your time and assistance.
Steve