12ml every 2 week isnít going to work, you should be dosing at a minimum 15ml 1-2x every week and changing 50% water every week.
If you want to do low range dosing you need to do it more often and on a regular schedule. Dosing pump should be in your future or your going to continue to fail. Low range dosing and only changing water every 2wk you are going to have to zero in on low daily dosing. Zero in on what your plants need every day or every other day with almost zero accumulation. Your tank is going to have to be automated on dosing and top off. You need to flatline your levels, you canít do that if your not there to manually dose on a reg schedule ó> dosing pump, at least a 3 channel for macro, micro, evaporation dosing.
What your doing currently some of your nutrient levels are bottoming out even before 1st week is up. If your going to continue down this path boost your P, K and micro dosing to front load them and hope your tanks natural biological ammonium/ammonia production will keep N from going into nitrogen depletion. For sure from pics you posted I would suspect your phosphate are on a rollercoaster ride of slightly to much to going into complete depletion before 2wk are up. Phosphate slightly high for a few days, not that bad, phosphate bottoming out for 5-7 days at a time, bad juju. Phosphate bottoms out and plant growth stalls and spot algae your seeing comes forward.
Seriously, your so close, even training wife/spouse to pour 1 gal of water in tank that youíve pre-dosed at wk1 between your wk2 water changes/attention would make a huge difference. That last week where some of your levels are bottoming out is what is causing the spot algae flare ups. I call it feast or famine dosing. One thing plants or even your bacteria bio-bed/colony or fauna donít like is constant fluctuations in levels.