I start by washing out the pre-filter and the polishing sponge in plenty of clean water, until the water ran clean when I squeezed them. I also washed the pre-filter basked under the tap to make sure all the plant leaves and suchlike were gone.
Next, I tip the bio media into a bucket of old aquarium water, give it a swirl and then catch it back out and place back into the basket. I repeat the process for all the media baskets, making sure that I work fast enough so that the media does not have time to dry out because the bacteria can not survive out of water for very long.
The last step is to drain and wash the filter cannister under the tap, to get the remaining debris out of it.
I place the baskets and the filter pads back into the cannister, connect it to the aquarium and use let the water syphon into the filter via the inlet hose.
By this time, I use up about 45 litres of water for the cleaning, which is enough for the 10-25 % weekly water change, so I top up the aquarium with cold dechlorinated water, as is normal. The temperature drop is about 1 °C, which is not even enough to trigger spawning, let alone bother the fish.
Test results are still odd because of the very large water change, which was the upgrade three weeks ago: nitrate 15 ppm, pH 7.4, 19-20 °GH, 3-6 °KH.