Hi,
Once the sponge started to turn white, you should remove them from the tank to avoid 'spoiling' your water. You can 'frag' the healthy part out with a scissor or pen knife and wedge it into a crevice where there is strong current... it will then regrow from there on.
To keep sponge in the tank, you need to find a good spot in the tank where the current is strong and preferably with wave motion - the latter will ensure that it grow upward; my experience with blue sponge is that if put in a one-direction current, it tends to grow weird extensions.
IMO, blue sponge is not easy to keep... if you want to start with sponge, get the orangey-pink sponge - it is very hardy and can adapt to different current... do note that it tends to attract algae if the current is not strong.
Regards...
Raiden