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 
From what I know, I think you should hold your altcoins. For optimism to the altcoin business too. If you sold it immediately, chances that youll sell it at a lower price, because price shifts either high or low may come. And dont be discouraged from low price shifts.