I have a 20gallon/70litre freshwater aquarium with two comets now about 2inches each.
The aquarium gets dirty with algae over the glass and all the plants very quickly after cleaning. This is nothing like the freshwater tank (same size) I had before where I'd have up to 20 smaller fish (guppies, mollies, platys, rasbora) and really never having to deal with algae due to healthy plants.
I feed them flakes once a day using just a pinch. I've tried to plant out this aquarium densely with all different sorts but many of the plants die off. I used to have 20watt light but doubled to 40watt to help the plants. I had this same problem before doubling the light wattage. The light sources are new so the light spectrum shouldn't be an issue.
Water quality is tested and readings are all fine. PH 7.8, Ammonia and Nitrite are 0, Nitrate was around 20ppm last time I checked. Tank is in a garage outside of direct sunlight.
Is there any plant likely to do really well in this situation and keep the algae at bay (by consuming excess nutrients).