I can see edges of the sandbed on the side of the rock in the full picture
It's got coverage, it has the extra growths and impacted waste possibly unless you've been dedicated cleaning it the whole time. This is classic old tank syndrome
I wrote a work thread on reversing old tank syndrome, by rip cleans of course
the cause of old tank syndrome is compounding whole waste particles that blanket the interstitial sand grain zones and plug up the channels in live rocks- in some cases it's sheer overgrowth allowed from various invasions that plugs up the surface area on live rock combined with a thin veneer of...
www.reef2reef.com
In this thread the university of Florida I think it was uses rip cleaning to rehab and skip cycle rebuild just after a hurricane loss of power crash. There's several examples of tank handling done our way saving them from loss
Notice the recurring themes:
We don't ID anything
We didn't ask for parameter readings
We never used bottle bac: keeping the cycle intact means we just cleaned the tank, it's not a start over. A start over is a new cycle on new rocks with new corals and fish. By surgically cleaning your tank vs guessing with dosers and test kit levels you skip the risk of giving your tank full blown dinos or an alternation of generations into a gha forest.
Your tank will simply look brand new 24 hours later and you keep all your current sand, rocks corals and fish. We need all new water matching temp and salinity only for the job.
R2r is where my examples are because of the rate of entrants = high but we want to build new work threads here, live time fix jobs. At fifteen years you've got beyond common time and invest into that system. It's 1% on the site to have a nano that old. There's a lot on the line.
Only choose a method that works in others tanks by pattern.
**side warning
Anywhere I make work threads that eventually become logs of tanks worked with outcome tracking, at the start there's haters. I have to endure it lol to get my outbound reef work. It's noise, static in the line, rf multipathing one must deal with. Ignore the trolls
Consider just the actual reefs: what did surgical skip cycle full cleaning all at once do for the tanks, were the owners happy?