hi bro,
nice to see so many pbt in 1 reef tank, the color is awesome.
very good write up on the hydra too.
just want to share my limited knowledge on tank stability.
i think most of us understand that no die off of fish or corals would mean the tank is stable. my old school knowledge to this is determine by, when a fish is added in. there is minimum or no ick on new fishes. sandbed is 90% or almost clean of diatom or cynobacteria or anything similar. Also most tank crash would come a couple of months later. It is when a system have too much to cope and breakdown.
no doubt that you have shown the system you have done is a success but there is more boxes to tick then what you have listed and the limited example i have given is not all. this limited knowledge is base on my personal opinion, no offense to you or anyone involved.