Fyi: 20L is not a typo - it is a 20 long. I'm not sure I understand what you mean by sugar sand? Would anaerobic gravel be bad ?
I was getting some cyano algae so i started adding some fertilizer the past two weeks. This picture was taken before i added the fertilizer - there is a little cyano in the picture on the left middle front and not shown in this picture a blob grew on the heater (pretty thick). It was easy to pull off as a single sheet and at that point i added fertilizer and did the water change. It also coated one plant leaves but that also pealed off pretty easy - took about 2 minutes and didn't stick at all.
I have this gravel in one other tank but black algae is growing on the substrate in that tank (that tank gets regular water changes - about 50% once a week and has a lower fish population (3 angles, 6 white fin rosy tetra, 4 kuhli and a sae) it is a 29. The nitrate in the 29 runs around 15 - hence the weekly water changes. Not sure why black algae grows in that tank (not on the plants just on the gravel).
Sugar sand is sand with really small grain size about like processed sugar you by at store, average grain size of sand is .3-.75mm, which is what I’m seeing in picture.
Being that small the pore structure in between grains are is very tight and oxygenated water has a very hard time reaching lower layers. When oxygen is low a different type of bacteria will form, anaerobic bacteria. They actually strip nitrates of their oxygen molecule to breath and turn nitrates into nitrogen gas which drifts back up through gravel into water, up to surface and dissipates into atmosphere.
The light blue/green algae forming on surface of sand around tank also supports that this is happening in your sand. They are a nitrogen fixing Cyanobacteria that uses the nitrogen gas being released from sand to grow.
And anaerobic bacteria has nothing to do with stocking levels as one poster alluded to. Only thing required to grow anaerobic bacteria is any presence of nitrates in water column and low oxygen levels.
All anaerobic bacteria activity is not bad, in fact it’s a important part of the nitrogen cycle in nature. Only in the presence of high levels of raw organics can it become lethal to your tank, you will not have that problem as your sand layer is so tightly pored almost all raw organic matter stays at surface where it is broken down by aerobic bacteria to nitrates. Anaerobic activity is exactly how Seachem Matrix and other nitrate reducing media like biohome etc work. You can actually build a denitrification filter if you want, people have been building them for decades.