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 
I can suggest such a strategy. Select the most promising altcoins and hold them. The rest of the coins are exchanged for the Etherium and participate in the new ICO, because now there are good projects If I were you , I better sell it so that you can use your profits to other investment. If you hold it , you can not enjoy any of your investment. Just sell it while the price is still good . Then buy more again when the price is Very low. That must be good stragedy you can do when you buy altcoin.