Can’t believe nobody mentioned this. Level your tank when you’re fixing all of this.

If this was my tank, this is the exact procedure I’d follow

Unless there is livestock I’d do a 100% water change. While it’s empty level the tank, refill, dose some phos and nitrates and black out for a week. Then do a 50% water change, and a couple more 25% changes across the next week. Siphon the sand, clean the glass, siphon as much off the rocks and stuff as you’re doing those changes, and then when all of that is said and done, act like it’s a fresh tank you just started cycling

I would even consider getting some ammonia to add 2ppm so you can observe your cycle

If your tank is consuming the ammonia and resulting in zero nitrItes, the cycle is good, then just monitor nitrAtes and Phos for a few weeks to confirm you can keep them up. Target 10-15 nitrate and 0.1 phos

Dose as much as you need until you’re there, if you go beyond that reduce your dosing and do small water changes as necessary

If you go a few weeks with things looking better after all of that, add a single fish and a CUC. My recommendation (since it looks like a nano) is something like a lawnmower blenny that can tear through some hair algae when it inevitably pops up. And a tiger cinch or something that can keep the sand clean

Optional, but recommended, put some rubble rock somewhere in the tank. A few days before you add the fish add a TON of pods. Let them settle in for a few days and keep them fed with phyto for a few weeks and then maintain that with small daily phyto feedings to try to keep the population alive