Ive had a few customers recently enquiring about pond vacuums and how to clean their substrate.
My first question to them is who told you to put substrate in your pond? The usual reply is that another pond shop has told them that they must have substrate to host the bacteria that runs their nitrate cycle. BRRRRRT wrong...
In a natural pond with no powered filtration system, yes you will need substrate to host your bacteria. These natural ponds have a large volume of water and a larger volume of plant life. They usually don't have koi in these ponds and these systems are rarely "crystal clear".
Most backyard pond systems use a powered filtration system that relies on a pump connected to a filter. It is this filter that hosts your bacteria. Putting substrate in these ponds does nothing but cost you money.
Unscrupulous pond retailers are happy to sell you small pebble substrates that your pump will happily suck up. This will damage your pump, void your warranty and cause you no end of problems in trying to clean the pond.
If you have an adequate filter, you do not need substrate and you can easily vacuum your pond with a pond vacuum and chemical flocculant like PondMax ClearMax.