its a "neutral medium" if soaked and flushed before use. if there's anything in there, you put it there. I like the simplicity of that.
watering can be tricky until a couple weeks in but its easy to weigh the blocks (wet and dry) to get a gauge.
they can hold a lot of water, better part of 2 kilos total weight for the 6".
never noticed more than about 100 g. difference block to block soaked.
6x block unused bone dry is around 250 g. with plug.
if you think you over watered set the block on a dry towel and watch it get wicked away.