The one that is getting worse instead of better I didn't notice for the first year or two. Then when I noticed something fishy I recorded the habitability at 2.9803. One year of terraforming later the habitability was up to 2.9853. Unfortunately I didn't write down the other environmental specifications the first time. Below is the only data I've recorded so far. The planet started with zero atmosphere and hot (it is the inner most planet) but due to mineral resources was still worth colonizing even though it was near 3x. Since there was no atmosphere at all I just started adding oxygen. Why would going from no atmosphere to 0.0103 O2 cause the habitability to go down? Is the O2 making the planet hotter?
Any increase in atmospheric density will increase the temperature slightly, although greenhouse gases have a much more noticeable effect. If the col cost is almost 3 then the temperature will be the major issue. Add anti-greenhouse gas to lower the temperature. Once the col cost stops falling at 2.0, the lack of a breathable atmosphere will be the main problem and that is the time to add oxygen.
Steve
This turned out to just be my not fully understanding the terraforming system. I switched that planet from producing O2 to producing the generic anti greenhouse gases and after 1.6 years the temperature had dropped 1.7C and the attrition rate started going down a fraction. I would like to suggest however that the 30% O2 being toxic rule not apply until there is a meaningful atmospheric pressure (or perhaps its negative effect be multiplied by the pressure or such). I know you are not going for perfect climate modeling but still, that tiny fraction of an atmosphere I had going at 100% O2 wasn't going to be breathed by anyone therefor it couldn't have been toxic.
The new stats which show the temperature dropping 1.7 degrees and the attrition improving a tiny fraction are.:
9/9/2030
Planetary Suitability (Colony Cost): 2.9342 (A decrease of 0.0511)
O2 73.5% 0.0135atm
Anti-GH 26.72% 0.0049 atm
Total pres: 0.0184
Base Temp C 134.14
Surface Temp C 140.99 (a drop of 1.7C after 1 year 7 months of terraforming (5? terraformers)).
Anti-GH Pressure 0.0049
GH Factor 1