It is unusual to lose everything if they all arrived alive.
I'm not too fond of the plastic Tupperware style brooders. They aren't really big enough for more than a few days for 20 chicks. The space can't grow as the chicks grow and I suspect, it was too hot. They only need one small hot spot like with a broody hen and the rest of the space should be cool.
In warm weather I brood the chicks in the coop. In cold weather, I use large cardboard moving boxes from Lowes or HD. They only cost about a dollar or so and you can double or triple them as the chicks get bigger.
If your brooder is in the house, room temperature, you don't really need any elaborate heating system. Depending on the size of the tub, I would say a 75 or 100 watt ceramic heat emitter in a brooder lamp fixture would be plenty. 20 chicks can huddle and keep themselves warm for the most part.
While it won't kill them, 16% chick starter is a bit low. I wouldn't go with less than 18% for baby chicks and I like to start off with about 20% for the first couple weeks. The younger a bird is, the higher protein they need to build their body and immune system.
The amino acid profile of the protein is important as well, but any manufacturer would take that into consideration in their formula.
