I've had some luck throwing some dessicant in with a 'solid' half bucket of salt. Since salt is hygroscopic (absorbs water, typically through humidity in the air), it ever so slightly melts the salt. Then when the bulk of the moisture evaporates, it leaves you with little salt grains that are 'welded' together. Desiccant takes out that last bit of moisture, leaving the salt-brick quite brittle.
Here's how I got learnt on the matter! It all starts with the usual, tank problem >water change > long hours> tired and forgetful by the time it's done. Left the bucket top cracked open to find a brick a few days later. I happen to have some silica gel beads that we use in our plasma cutter machine, and put them in a mason jar with the top left open, and placed those in the bucket and closed the lid. A week later, took out the jar and the salt still appeared rock hard. I gave up and went to salvage just the plastic bucket by dumping the salt-brick into the trash can. As it slid out in one piece and hit the bottom of the trash can, it broke into about half fine powder and half chunks. D'oh! I could have used all of it after all.
Now when that happens, I repeat the process and just drop the bucket from about 1 foot off the ground a few times and it comes back to life, in bite sized chunks that are perfectly usable. @ $70+ a bucket nowadays, it was worth the $20 in (reusable) desiccant and 5 minutes of my time.
-Jeff