Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
As of today, the best thing you can do is just to hold your coins because the prices are still low but as the time goes by, you should sell your coins if the prices are increasing.