Yep. Sad to say but you "screw this up majorly". There is no way to earn by borwing only. You know that
You borrowed 30 eth, you need to pay back 30 eth (no matter ho much they are worth), you didn't do anything with those 30 eth so there is no profit for you. Just like borowing money from bank and leave them on you bank account without touching them. You should borrow USD (or BTC depend if you want to play on BTC/ETH or USD/ETH), buy eth, than sell ETH with profit, take profit and give back USDT (or BTC).