I'm not sure if anyone else mentioned this, because I made this mistake a few weeks ago when I set up a tank that was in storage.
DONT USE SOAP!!
That vinegar idea sounds good, someone told me to use bleach but everything has to be completely dry before you fill it back up with water otherwise the beach will get in the system but when dry does not leave residue. I used rubbing alcholol. Good luck bro. I priced out a 210 gal
system and it would run you about $2700. emtpy.
Personally. I would empty everything out and probably not put most of it back. Its more fun to watch it grow from scratch.