1- You said %90 of your live rock is 6 months old.Considering that you bought it from LFS - it is slower but die off inside newly shipped LR may last 3 to 6 months. OK water parameters don't show that cycle is totaly over. Living organisms have organic phosphate. Sand bed and anaerobic places inside LR are good harbours for phosphates but at he end it is thrown out. Thats why it is not advised to add SPS before 6 months.
2- These are the fishes that i saw in your pictures;
4 chromis, 1 sixline wrasse, 1 damsel, 1 mandarine,1 orange shoulder tang,
1 Kole Tang-probably- , 1 ocellaris clown. Did you keep them all at the same time in a 30 gallon? Don't get me wrong i am not trying to be a tang police this is not a proper tank size for tang issue. But those guys are swimmers. Even if they are small they consume alot of oxygen , add alot of CO2 into the water, eat alot and poop alot.
3- Protein Skimmers like Deltec, ER, Schuran etc. can eliminate the particulate matter as well as organic matter.Coralife skimmers may be good for their price but they can only eliminate the organic matter. How do you get rid of the particules in the water that create nitrates and phosphates?
4- Let me bash your sand as well
Your sand does not look like aragonite. It is gray and coarser than sea floor grade. I don't know what type of sand it is but some types of sand leach slicates in to water and cause excessive algae growth. Also coarser sand is a detritus trap. Thats why crashed coral is called as detritus trap.
5- Whatever i can see from the picture your rocks are piled in the centre. You have two spray bars on the sides and a Hydor coralia close to the surface. I believe you have a good water flow around the rock work and corals. How do you get rid of the crap from the rocks accumulating at the bottom. i kept a bare bottom 65 gal tank with 700 gal/hr return pump and a 2500 gal/hr closed loop pump. It was hell of a lot of flow for a 65 gal. Despite that the strong flow i had to syphon out lots of crap every week. It is hard to figure it out with a sand bed but too much crap comes out of LR, for me i is worse than fish excrete or excess food.Hobyists having BB tanks must have observed the samething. If you don't have any kind of mechanical filteration where did all that crap go?
My guess; if you kept all that fish at the same time you added too many fish in a 30 gal before the rocks fully cured and the tank matured. You tried to handle that bioload with a CL skimmer.It looks like you don't have anything to elimiate particules. If you don't have any flow pushing the detritus under the rock work and your sand is as coarse as it seems in the pictures probably all that crap was trapped by the sand. Nitrate and phosphate levels look allright but i would think about if this is because of the efficient filteration or consumption of GHA.