Dear BK Buttikoferi
This is a the way to fix it.
There 2 main reasons
Algae bloom. (if its green)
There no way around it, you need a uv light. Its happen from contamination. One drop that all it need. Even from the air or direct sunlight. If you go for total Black out, I mean total, no peaking for 2 week. it's usually 50% chance it will work, If your lucky. Just go straight for a uv light. For 45$ you can have one.
Foggy, cloudi (white, grey)
Your water has lots of particle in the water. Dose particle are attach to bacterial colonies. Its a normal process in aquarium, but because you have to many Decaying plants, excess food or unclean filter, you can see it. Over time that cloudiness will resolve itself. But water change will help to remove they excess particle.
Also, the filter will not remove any of it because its to small. You could add a water clarificater. But make sure to add lots of cotton(forgot the name) in your first stage filter of your aquarium. Better yet, use a secondary filter that’s easy to clean. Don’t forget to turn off your main filter, so that you don’t need to clean it again. Its usually completely clear up after 4 h.