30% in what timeframe? Daily, monthly, yearly?
From what I remember, the biggest crash days from 2013 onwards were:
- April 10, 2013: about -60% (the big crash from about $260 to about $120)
- December 6/7, 2013: about -25-40% depending on the definition (from $1100 down to ~$830, but had lows under $700)
I don't remember other days where BTC lost more than 30% per day. Even the January Crash in 2017 was less than 30% in a single day (the March crash, a.k.a. "ETF crash" slightly may have surpassed it), and the June 2016 crash (the "halving crash") was even smoother, but more sustainable because the highs were not reached again until late 2016.
For longer timeframes, look at
https://bitcoincharts.com/charts/bitstampUSD