I've had me tank with Guppies running for a while now. I've noticed that the inside walls of the tank needs cleaning because some green stuff is starting to grow on there. I've seen the magnet thing you can use and pads on a stick to wipe the glass clean. Hows about a new clean scouring pad on a sponge from the kitchen cupboard? It'll be a new, out of the packet, never been used before job. If it's ok to use I'd keep it with me water change kit so it'll never be used for anything else OR would it be better to use a new one every time?
Al
Al