Do you choose to hold or sell? I choose to continue holding.
There is an even better option which is to buy bitcoin with that huge discount, when you see that you should not react like, oh Im losing money, the right frame of mind is, oh Im going to get cheap altcoin, it is incredible that a lot of people get that reaction if they see a video game or a piece of clothing with a discount but they do not get that reaction with bitcoin. In that case, we could really use our bitcoin to invest with altcoins for the main time if bitcoins fall is at 20% a day. But I dont think so that it will be possible for now to happen falling that worst percentage. You need to be careful sometimes of choosing those altcoins to buy with, because more there were types of coins that isnt in demand and you could possibly drown your investments into falling value in the future that needs more months to increase its sell price. So better check the change frequency of an altcoin so that you eventually determine its trading activities, which could aim better progress.