Mine looked similar to that at the beginning too. So long as your soil doesn’t contain too much wood (floats and heaps of tannins), fertilizers (toxic), or non-organic additives. I capped with black quartz sand (mainly) and white river sand (for decorative purposes). The sands don’t need to be washed if they are from an aquarium supplier but doesn’t hurt to wash them if you are unsure. I placed between an inch (2.5cm) to 4cm of soil and 1-2cm of sand to cap it. When it looks dirty (like yours), change all the water but place a bowl under the hose to prevent the soil from being disturbed. I planted my tank when it was still dirty but many do it after the first water change. Good luck. This was how I did it and I’m sure there are many ways to do it.