Dirty sandbed

[SolarReefer]

Hello everyone. This is my sand bed. 6 month old tank. Looks like diatoms on top (which is new) but under the sandbed is tons of green algae and those weird lines. I added a diamond goby and noticed my nitrate spike up to 70 so I blamed him for that. Now I believe he died because he is now where to be found and my nitrate is testing 75+ because the Hanna checker only goes up to 75. I have to lower nitrates and I believe I need to clean the sand and then do a large water change. Could anyone steer me in the right direction so I don’t make a catastrophic mistake??

 

[TheBear78]

That's an awful lot of sand.
I expect that the algae lines visible through the glass are only on the edges.
If it were me, I would remove probably half of the sand (assuming it's that thick throughout) during a water change. That way I could syphon it out with the inevitable muck that would be unearthed.
I usually run a finger along the edge of my sandbed when I clean the tank which loosens most of the algae on the edge. It rarely needs scrubbing off of the glass.