Fish Poo

Looking at the photos on your other thread, you have plants at the back but a plant-free area at the front. But I can't tell from the photo if there is gravel or sand on the bottom of the tank.

If it is sand, just suck up all the poo that is lying on top of the sand. Hover the siphon tube half an inch above the surface and make little swirls to lift the poo off the sand and into the tube.

If it is gravel, you need to clean the non-plant areas; at the back where the plants are it will sink into the gravel and feed the plants. Clean gravel by inserting the gravel cleaner right into the gravel, then when no more poo goes up the tube, move the tube to the next patch of gravel, and so on till you've removed enough water. If you hit roots, move somewhere root free.