Thank you for replying, every little bit of help is welcome!
I am looking into overdriving the tube and/or adding more lamps but I thought it would have been wiser to sort this thing out early on.
The tank is still pretty new as it's been populated less than 2 months ago after the obligatory fishless cycling. I was advised against adding any fert during the first weeks as the plants would not take up any nutrient.
No matter the low light, plants are actually growing and putting out a good amount of new roots, especially the two anubias nana which would also put out a new leaf from time to time. That's why I started dosing 0.8 ml daily.
This number come from the recommended dosage for FP, which is IIRC 10 ml every 20L (5g) first time and then 5 ml/20L every month --> hence the daily 0.8 ml.
Believe it or not, it would become slightly green even even then, although FloraPride is, as you've already noticed, so much close to glorified water that one would feel entitled to wonder how it could possibly make any difference.
I experimented with 0.8-0.2 ml daily but new holes were appearing, then I tried squirting in as much as 15 ml of FP, and water got even more green - I wonder how it would like after pouring a massive 70 ml
Probably there's some imbalance in the nutrients, is it worth trying a switch to the Seachem line?