Salifert are reported to be more accurate. But I think your API should be fine. I do all of the tests personally and then log it into a spreadsheet to help me keep track of my levels. But then again I am a computer geek (yes I do it for a living as well) and pretty anal so this is normal for me.
I think the green weeds is possibly hair algae. The black spot on the base rock almost looks like diatoms as well. It is very possible that they could have spread to the base rock.
I have looked at my temps with and without the lights and they are pretty constant around 79. I have to agree with Biff, you probably need a new heater. Does your house temp go down at night? If it does, you may want to change your thermostat to stay at a more constant temp.
I've heard that Jager are really good heaters. Personally I just have a little Marineland and it seems to be doing well for now. I'll probably upgrade soon to a Jager and just use the Marineland for mixing water.