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 
If you want more profit in the future then it is best to hold. Now that bitcoin prices and all altcoin are very low, if you sell you will suffer losses, instead you should buy more. I believe that at the end of this year bitcoin will rebound and reach high prices, I hope that the end of this year Bitcoin will reach $ 30,000