Just remember it's all about balance. In your last picture, it's obvious there was just too much lighting on as the plants were dense enough to make use of it + the CO2 = more algae. It happens... especially early on when the plants aren't established.
As for your large tank, just look at it as a new start. You have a large tank... going for cardinals isn't that exciting (at least not to me). Try a different breed... roseline sharks are beautiful for example.
What's alarming is that you said your CO2 didn't turn off... but people here leave their CO2 on all the time. Less is more in this case. Less = plants alive, fish alive. More = more algae, dead fish. So it wasn't that the CO2 didn't turn off, it's more like you just put turn it on too much and they suffocated. It should never be on so much that if the plants aren't producing O2 (at night for example) that the fish die immediately. But you're almost there and you have all the equipment so I hope you have better luck next time around.
The hardest part of this hobby is patience - "PLANTS AREN'T GROWING 10 INCHES A DAY? CRANK UP THAT CO2 AND PROFIT" is the wrong mindset. Patience patience patience.