I have a 350 gallon plastic potable water tub that held saltwater for many years. It has been dry now for about a year, and I would now like to start using it again. It has hard crud all over the bottom. The problem is that the opening is only about 8 inches wide, and I can't get in to scrub it. I have tried a scrubber on a long stick, but it is useless because I can't reach the sides and ends, and it is impossible to apply pressure except straight down. Is there anything that is safe to use to totally loosen the crud and then can be flushed out so the tank is clean? On the other hand, the crud is all saltwater mix buildup mixed with normal dark dust accumulated over time, so would it be safe to just leave it there? I would rather clean it. IF there is no negative residual effect that can harm a reef tank. Thanks.