quickly place your thumb over the tube for a few seconds. This will stop the flow allowing the sand to return to the bottom of the tank. Releasing your thumb will commence the syphoning. When you have gone over all the substrate like this, keeping a close eye on the amount of water taken out,(not more than 30%) stop the flow. Then replace the water with fresh (conditioned ie. de-clorinated) and you have cleaned your substrate as well as completed a water change!Safe, thank you very much.
Can you also suggest i good plec (hardy) for a 4 week cycled tank?

if it is not in your in box, check your spam filter, I bet it is in there