Hi Sandy,
Look, you can fix this, one way or another.
There are a number of issues though. What is causing the nitrates in the first place? Is there anything in your system that uses Bioballs, Biowheels, Bio-anything? Do you have sponges in the overflows, or pads in the sump? If the answer is yes, these must be removed.
To lower nitrates, you must change vast amounts of water. Quickly. If your tank is 100g and the Nitrates test 100, you'll need to do a 50g water change which will make the nitrates 50ppm. Do another 50g water change, and you'll get them down to 25ppm. So 100g later, you've reduced the problem significantly.
Your LR must be blown off thoroughly to remove detritus. Overfeeding could be part of the problem. A lack of circulation could be another part. How often do you clean the skimmer's collection cup and riser tube? At this point, do it daily. Yes, all this is extra work, but you'll bring your tank back from this situation to a happy tank full of healthy livestock.
Another thing you can do to remove Nitrate is to purchase Algone (
www.algone.com), which is a product you toss in the sump for 14 days at a time. It absorbs nitrates. Each 'pillow' is rated for 200g, and a box has 6 or 8 pillows. It doesn't work fast, but it will work around the clock when you aren't doing water changes or cleaning that skimmer.
You mentioned HLLE for a couple of tangs. They can be healed in a quarantine tank. Yes, more work. You've going to have to really invest some time in your tank for the next 4 weeks or so, but all of this hard work will pay off, I promise.
You can run fresh carbon in a canister filter to help pull nasty stuff from the water. You can run phosphate remover in a Phosban Reactor if the PO4 levels are high in your tank - which they probably are based on your initial post. You'll need to test the water for phosphate of course.
A refugium will assist your system, but you need your tank set up correctly in the first place if you want it to make a difference. The refugium should be sized to be 10% (or larger) of your display tank. My 280g reef has a 29g refugium.
Feel free to reply with your thoughts, and I'll try to help you get this resolved.
Btw, since you have a service guy, he can do these water changes which will save your back.
![Wink ;) ;)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png)