Should I remove my shallow sand bed - issues in tank

[corpusse]

I'm having some issues with both nitrates and phsophates. Nitrates are 20ppm+ and phosphates are .2ppm RO/DI was tested at 0.00 so it's not a water issue.

System is a 180 and a 40 gallon plumbed together + sump. There are currently no fish in the system as I am treating for ich. I have not fed the tank in weeks but cyano is growing like crazy in the 180 only. The 180 has 1" or less of sugar sized sand. Sandbed is about 14 months old. I had to tear the tank completely down to get all the fish out. Stirred everything up, rocks out of the water ect. I hadn't tested phosphates in a while but assumed wrongfully they were okay as I frequently change gfo and my feeding habits have not changed in many years. Nitrates have always been somewhat of an issue despite the fact I only have about 15 fish (2 tangs rest small).

The 40 gallon has never had any cyano issues but it has a haddoni anemone so there is a 6" dsb. The 40 gallon is actually low flow so it's not a flow issue in the 180, lights are all led's so no old bulbs.

I have done a 75 and 85 gallon water change after removing the fish. Neither had much effect other then rising the alkalinity. I can really only make 110 gallons of water max at a time but I am thinking I might just have to get some more barrels and do a 100% water change and just deal with an alk shock.

I have tried nopox and it did not work, but I don't believe this phosphate issue is long term which is perhaps why the nopox did not work. I am perplexed how it's gotten so bad. I grow 2 different kinds of macro in addition to the gfo. I also had a 15 gallon barrel of sand plumbed in which I thought was not helping nitrates but may have actually been keeping the nitrates at about 10ppm as after the barrel developed a small leak and I took it offline they rose.

My fish are going back in november 6th and I'd like to solve this by then. I do have NPS corals I am eventually going to have to feed again too. So I am thinking the SSB might be the issue. I could remove it despite my dislike for bare bottom tanks. Next year I plan on moving and I could deal with the bare bottom until then and just place some sand containers for my wrasses to sleep in.

Otherwise I can try nopox again, maybe give biopellets a try, buy a ton of gfo and start changing it out every 3 days, or more barrels and 100% water change. I welcome any suggestions, I figured the tank would be on cruise control as I worry about controlling ammonia in the tanks my fish are currently in but no such luck.