In a 55g u can use 2-3 pieces if they are not bigger than a zippo lighter. Just test the water for a while to make sure. In 150g... i think you can throu all your teeth, bones and whatnot. 10 teeth can't do that much damage to 560 liters of water.
Still, start with 1-2 pieces( from 55g up) and test the water for pH changes. If you see none you're safe.
But to be even safer just take a bucket(the standard 10 liter one), fill it with water, test the pH, put 2 teeth in, test pH again in 1 day. If it doesn't shift then the 2 teeth can't do damage in 75 liters(20g) either.
You can also add more each day and see at what point the pH changes in the 10 liter bucket. You might end up with 2-3 teeth, some bones, etc. and if the pH is the same in the bucket after...3-4 days i'd say it's safe to use them in the tank to.
OR you can go the really hard way and add wood that releases tannin to help drop the pH and so the teeth raise it and the tannin drops it and you can find a ballance, or use co2 to make the pH drop(this last idea of course is hardcore stuff and can be included in the do not try this at home unless you're good at it category).
And now i will stop posting before i encourage you to make even crazier stuff