I have been reading your thread for awhile, and I really admire that everyone on the forum has been trying their bests to help you and your tank out. This forum is a great forum with many good people.
I think what you should do right now is not worried too much about your live stock. Just focus on how to control the nitrate and phosphate in your tank first, then everything else will follow along.
You see so many bad stuff, algae, and cyano in the middle of your tank, on sand bed and on the back wall because there is no or very less flow at those areas. Try to improve more flows on those areas, then you will be fine with that. What kind of powerhead/wavemakers are you using right now?
You said that you use the canister filter for your 125 gallons tank. What kind of canister and how big? In my opinion, the reason you have had so high nitrate in your tank is not enough filtration. I 100% agree water change is the best way to maintain the tank and control the nitrate, but if the filtration is not good enough, then it's useless at some point. Unless, you do daily water change to remove all the nitrate out.
Have you been using RODI water to mix salt when doing water change? Before you want to deal with phosphate issue, you should know where the phosphate comes into your tank first.
You don't need to dose anything for your tank at this moment. Just look back, think again, and slowly but surely do one thing at the time to figure all the problems, then find solutions for that. It's easy to say, but very hard to do. I understand that, but you have this forum with many great people here to help you out. Just don't listen to your local store about anything anymore. They might be nice to you that not taking your big money at once, but they know how to play with you and get your small amount of money here and there slowly.
Putting stuff from ocean/beach into your tank is 50/50 because you don't know what they carry with them. Everything can balance itself in ocean well, but in closed system is a different story.
Hope this helps