I've had a 40L tank for 5+ years now (small flat so can't go bigger). I've always struggled to keep plants. A few months ago I did a complete overhaul - new Tetra complete substrate, gravel, new Interpet PF2 filter, new decorations, Echinodorus plants & moss balls.
Unfortunately the plants are dying off and I am getting cyanobacteria & brown algae on the gravel. API test kit results are all fine.
I dose API leaf zone (5ml) weekly & API CO2 boost daily (1ml).
Light wise, I have a LED tube suction mounted internally on lid (can't use any side mounts). Experimented with just a few hours of light to all-day, seems to make no difference. No direct sunlight.
Not sure what else to do really, its super frustrating. Currently only have a couple of tetras in there, looking to get a few more and couple of corydoras in due course.
I wish I could help but unfortunately, although some thoughts strike me, I don't have sufficient plant knowledge/experience to be able to help you comprehensively and so I hope a plant-keeper may be able to offer their views.