Biodiversity definitely does not decrease, what will decrease possibly is certain microfauna that something in your tank is eating, a healthy tank should still be able to maintain a level of that microfauna, I don't think anything could totally make extinct any type of microfauna in the tank? Also maybe people get "pod bursts" which then seem to "diminish" but more likely they find refuge in the rockwork, sand and sumps.
I could see it reaching a peak, and slowly declining to leveling out. But never decreasing for all time?
I know that wasn't the point of your thread but just wanted to talk about that, not sure where you heard it decreases long run? Additionally since you're addressing "new tank syndrome" there's absolutely no way you would ever see a statistical graph of scientific measuring of biodiversity in a tank decreasing within the first 1-2 years. It would only increase in that time, thus definitely why a lot of people talk about it helping handle
CERTAIN new tank syndrome situations like certain algaes, and dinos, etc.
Additionally every single tank has it's own situation. I think there are TIMES When a tank could have toxins being released, but is that happening with every single set up, I doubt, I think it's few and far between tbh. I don't think the leaking toxins from PVC and Glue on plumbed set-ups has a big factor.
It more about the overall surface area becoming coated with living bacteria and the overall "Ecosystem" which is completely separate from the "Nitrogen Cycle", some people mix up an established system vs a system capable of sustaining life.
Could there be certain unnatural elements in the water that are stunting bacteria, microfauna, and other growth YES ABSOLUTELY. I've thought that myself many times.
Some of the Expoy's and Cements we use maybe as well are getting wet before fully curing therefore releasing trace amounts of whatever they're made of into the water, it's hard to find a composition list of exact ingredients for some of the reef epoxies we use. But then again that's all speculation. Someone would need to do a lab analysis of the water throughout the whole process. But is that actually a problem. Who knows?!? It's all about you being able to spot, and just kind of "know or feel" with the force if that is really what could be going on.
There's no way you could take a dallop of sand from a new tank, vs a 2-5 year established tank, and not find the sand in the established tank with 2x or more life.
I think it would just apply to the entire surface area.
I don't think we will ever completely understand the atomic and microscopic changes that a reef tank goes through within the first 6 months to a year. You'd have to set up a reef system in a completely lab tested environment with many sets of microscopes in different areas with a constant monitoring and ID of everything going on on a microscopic level.
No one's gonna spend that money for this hobby at that time. Maybe if we found something in saltwater that cured cancer

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