I think this is a very difficult question to answer and can only be based on personal experience.. And also relative to the quality of a piece of wood.
Question you can ask, where was it stored and how long was it there before you put it in the aqaurium. An example a LFS near my place buys 1m³ crates with aquarium wood and stores them outdoors. To sell 1m³ of wood takes them quite a long time, after almost 3 years i still see the same crate standing at the same spot not as full as it was but still.
Now i wondered, he ordered a crate of wood.. The importer he got it from probably stores it the same way.. Crates, likely more than just one and likely outdoors as well.
This storing outdoors in all weather conditions slowly deteriorates the wood it gets softer from the outside in. It still can feel hard, but still once it is constantly submersed this process will only accellerate and leach. No saying how thick that softer leaching top layer is and how long and how much it will leach.
And than it is called Savanna wood, Mopani or Opuwa? But what is it? In a Savanna a number of suitable tree or shrub spieces could grow.. Mopani and Opuwa? I never realy found out what tree this wood is from, both are Districts in Africa where the wood is gathered by locals and sold to exporters. Best guess it can be from a number of tree spieces..
Spiderwood? Seems to be a root piece and part of a stem from different suitable Rhododendron spp. Which one, beats me, it says spiderwood. found this the fastest deteriorating wood in the tank.
Bogwood? Came from the peatbogs, a naturaly processed piece of wood dug up from the peatbogs, gathered and piled up at the bogs edge and when the pile is big enough crated and so on. No way to determine whit the naked eye what it realy is other than a piece of bogwood. You can though it and you can smell it. Does it feels soft, skip it find one that feels hard but than still you are in for a surpise..
So there are just to many questions impossible to answer because you just can't know till you throw in the tank and see what you got. Than only water changes will help and or filtering over carbon. But even carbon never gets it 100% clear.
Anyway even aquariums without wood, the water will gradualy stain. Than there is light and in a very warm colored light source towards the yellow color, than staining will show much more obvious. A cold light color towards the blue will look significantly clearer. But if you put the older aquarium water in a white bucket next to a white bucket with fresh tap water you will be surprised to see how stained it actualy is. But in the aquarium a cool white light it looks very clear.
So fooling your eyes with light color defintively helps you forget you are actualy looking at stained water.
And that's a very good thing, since tanins are prooven to be vital for the health and longevity for everything that lives in the tank from bacteria all the way up to fish.
So that's also another question a fishkeeper should answer for themself. What is more important? Tanin stained but rather healthy water? Or taking something vital out for my personal cosmetic prefernce?
