What's my tank problem? Need help :(

[still001]

Hello,

I'm new to the world of planted aquariums. I have a tank that measures 50x30x30 cm. My tank only has a sand substrate and no fertilizer or plant nutrients. I use a Week Aqua light that's on for about 6 hours a day. I'm having an issue where the leaves of my plants are turning black, as you can see in the attached photo. Does anyone know the cause of this and how to fix it? Thank you.

 

[MaloK]

On some pictures it's difficult to see if the tank has black beard algae. If the leaves are going black, you need more iron in the water.

Potassium, magnesium, or manganese are also probably too low.

From your pictures your light does not seems that weak.

 

[still001]

On some pictures it's difficult to see if the tank has black beard algae. If the leaves are going black, you need more iron in the water.

Potassium, magnesium, or manganese are also probably too low.

From your pictures your light does not seems that weak.

Thank you for your response.

I'm using a Week Raptor M450 light at about 60% power. I think the lighting is probably sufficient. The plant leaves are gradually being covered with black patches. How can I tell which nutrient my water is lacking? Is there a simple way to measure it?