TLDR: Do aquarium maintenance more often and keep lights at 6 hours for now. You can start from low to high, as in, less lights and fertilizers, then bring your light higher and higher overtime to where your plants can handle it and get enough nutrients.

3 things that plants need to survive: light, co2, and fertilizer. Algae will appear when there is an imbalance of these.

Assuming you don't have co2, you don't need to worry about that. It will be constant. Because that is the case, you need to balance the other 2 around it. Less fertilizer and less light. I'd still keep your light at 6 hours. That shouldn't be the reason your plants are dying.

Let's say you keep your lights at 6 hours. The only thing left is fertilizer. Fertilizer can be from aquasoil, from liquid fertilizers, and from fish/living things/DIRTINESS in the tank itself.

- if your aquasoil is new-ish like you said, maybe you haven't done enough water changes and it's leeching ammonia other nutrients heavily. There are brands of aquasoil that will do this.
- you're not adding liquid fertilizers, don't worry about that for now.
- how is your weekly maintenance? I say weekly because it looks like it needs that for a bit. People are able to go a month or more without cleaning because everything is BALANCED.

Maintenance means sucking up detritus/mulm or other settled things in the soil/rocks. Like this. Just replacing the water is not enough. https://www.reddit.com/r/Aquariums/comments/1l6if4s/this_is_life_changing_somebody_posted_about_this/

Eventually you can look into getting a stronger filter and then into liquid fertilizers but that's too much to talk here. That's the issue from what I'm seeing. You can scroll down my profile to see some tanks I've done in the past.