Your tank sounds like it's Phosphate deficient as mine was also. I had the exact same issue as you and struggled with it for well over a year. My nitrate level was not that high but was at 50 and I couldn't get it to move for anything. I dosed everything I could think and had a large Refugium full of chaeto. My tank was that way from the initial startup and would never change.
I dosed vinegar, biodigiest, aquaforest pro bio s & np pro, vsv, etc. I was running gfo and doing weekly 20% water changes. Occasionally I would do a few large ones every couple of months.
Nothing was working. My PO4 was 0 but I couldn't get my nitrates to budge. Most of the filtration methods listed above can easily and rapidly knock out any PO4 in the tank but are not as effective as quickly with your nitrates.
I finally ended up going to a very experienced local reefer for help. He explained to me that my lack of any detectable PO4 was actually inhibiting the reduction of my nitrates. Nitrates/PO4 have a relationship in a way that is similar to Alk/Ca. If one is very low while the other is high it inhibits the ability of the low parameter to be raised and vice versa. There needs to be a very diverse population of all different kinds of bacteria in your tank. Not all of them tackle the same problems. There are some that primarily reduce PO4 and others that go for Nitrates. Back then I had a very skewed bacterial population that was heavily geared towards PO4 consumption and the lack of that element in my system was stunting the growth of the rest of my bacteria population needed for nitrate reduction. You are in this same boat.
The plan we came up with for me that I am sure will work for you is this.
1)Remove all excess filtration methods that work primarily on PO4 reduction like GFO, macro algae carbon dosing, etc.
2) dose a product like phytofeast to actually elevate your PO4 and keep it elevated. Shoot for a PO4 level of 0.05-0.10. Figure out how much phytofeast you need to dose daily to remain in that range. Start by dosing ~ 3ml of that per day until you start to get a reading on your PO4 test kits. Adjust your dosing as needed to keep you in that range.
3)at same time dose biodigiest. For first 3 days dose it daily at 2-3x th e recommended value for your tank size. This has the best blend of the bacteria you are deficient in. After your first 3 doses scale back to recommended doses daily.
4) it took me about 3-4 weeks of keeping my PO4 elevated in this range before I saw some results. Your results may be sooner or a little after but you will all of a sudden... seemingly overnight see a significant drop in your nitrates once you get that bacteria population established. Mine went from 50 to 25 to 10 to ~8 within a matter of a week and a half.
5) you will need to keep dosing the phytofeast to keep your PO4 elevated for a while after your nitrate levels drop or else you run the risk of starving off the bacteria you just worked so hard to grow but you can cut your biodigiest doses to once every other week- per month.
You are looking to get your rocks to absorb some of that PO4 so it can slowly leach it back into the water over time. At that point your daily feedings and bioload will account for the remainder of the PO4 needed and you can stop dosing phytofeast all together.
Your rocks act as a sponge for PO4. They absorbs the excess in the water column and as you lower your free amounts in your water column it will start to leach it back out in your water until it balances out with the levels in your water column. Once something like gfo reduces the amount in your water column, then the rocks leach more out until equilibrium is reached again and so on and so on.
Most people in this hobby fight the opposite scenario. They have so much PO4 bound in their rocks that they play the equilibrium game I mentioned until finally the levels within the rocks are depleted to the point that the final equilibrium amount between the rock and water is at the range they were shooting for.
You will experience some Dino's or algae along the way but that will go away on its own as you correct your problem.
Hope this helps and good luck to you.
