Hi Phish,
Depending on the size of your fish, should determine if you need to chop the BWs up. My angel fish and Discus have no problems. For the rainbows with the narrow throats, I do chop the BWs up for them. I buy the block rather than the cubes, and its easy to shave off what I need that way. Ive tried cutting up the cubes.. and end up chasing the cube all over the place lol...
Now I just shave it all when I feed the fish the BWs.
Once a week treat for you fish would be good.. or even twice a week is fine.Guess it really depends on what fish you have in the tank and their dietary needs. Its a messy meal and you can always feed them the BWs just before your plans for a water change.

~ kate