Would this maybe help?
The poly-varnish method is as follows: -
1. Thoroughly wash the bogwood under a tap, sterilise if necessary, and then leave to dry for a day or two.
2. Visit your local D.I.Y. store and acquire a product called 'Polyurethane varnish paint.' This pure clear varnish will lock in the bogwood tannin. Please note yacht varnish is also sold for this process but this should be avoided as it will harm all types of aquatic livestock. (It is actually manufactured to prevent mussels attaching themselves to boat hulls).
3. Use the poly-varnish once then leave to dry for a day, and repeat this for another two days (three days altogether). This makes the bogwood airtight.
The effects are no fungus, no release of tannin and no worries about alteration of pH.
As with all things that are good at face value there is a setback with this process in that the bogwood can no longer produce microorganisms or 'lignin'. Lignin is particularly important to many species of Loricarins (catfishes, particularly Panaque species) as it aids their digestion and meets certain nutritional requirements science does still not fully understand. Microorganisms add beneficial bacteria to the gut processes of many Loricarins also.