Well I'm no aquatic plant expert by any means but did have success when I had planted tanks. I did have a Co2 system going and as for unsightly co2 tanks in plain view...there are smaller 5Lb tanks available which can be tucked away.
I'm not sure in what ways you were unsuccessful but did you have the proper lights? Many mistakes are made when plant roots are buried to far in substrate and wonder why the rot. Amazon Swords is one that fits this cause to die off.
To stay in line with your question...YES co2 dump is very possible. Automated co2 systems can fail where needle valve on the controller can stick open and if it happens during the night when lights go out and when plants release co2 you may end up with a tank full of dead fish in the morning.
I'm hoping the plant geeks chime in to offer advise on current systems on the market today as I've been out of plants going on 10 yrs now.
...Ralph