They now head to Parc Y Scarlets on Friday night to take on Simon Easterby's table toppers in the next round and Tandy admits the champions need to react quickly.
Tandy told Wales Online: "It was obviously a really disappointing night for us.
"We turned the ball over too cheaply, the set piece didn't function and we conceded real cheap tries. With the basic errors we are making, we are not going to win games like that.
"We got exactly what we deserved out of the game, which was nothing. It's the most disappointed I've been since taking charge and I know the guys are really disappointed.
"It's great that we've got over 9,000 season ticket holders, that's fantastic for us. The product we put on wasn't the show they deserve.
"It just wasn't good enough and that's not acceptable. We are not going to hide away from the fact.
"The accuracy and what we delivered on the field was far away from what the supporters deserved."