Why should we use pond water or rainwater to rinse the pump and filter rather than chlorinated water from the garden hose?

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Goal

How to Clean a Fish Pond

    Having a garden fish pond is awesome, but cleaning pond water can be a huge struggle. The following are several tips on how to keep your pond clean.

Materials

You'll need the materials below.

• Pond cleaning net
• 5 gallon bucket
• Rubber gloves (optional)
• Large container or bucket (if removing the fish)

Steps

Follow the steps below.

1. Skim the water surface. Use your pond cleaning net to skim the top of the water, remove any floating leaves and other debris from the top of the water.
2. Remove the pump, plants, and decorations. Removing the pump (be sure to unplug it first) as well as any plants and decorations from the pond will make it much easier to clean the sides and bottom. At this time it's also a good idea to remove any dead leaves from your pond plants.
3. Remove the fish (optional). Again, this is a totally optional step. If the water is really dirty, smells bad, or there's a lot of muck on the bottom, then I would recommend removing the fish before you start the next step. For light pond cleaning, you can just leave the fish in the pond and safely skip this step.
4. Clean the bottom of the pond. To clean the bottom of the pond, use a pond net. Simply drudge the bottom of the pond, and scoop out all of the leaves and muck that you can. Be sure to get into all the corners and clean off all any plant shelves, too. You could also try using a small pond vacuum cleaner or pool skimmer if you have one available to you.
5. Remove overgrowth of algae. Most pond water algae is beneficial to the fish and the pond ecosystem. But, if there is an overgrowth of algae, then you'll want to clean some of that out. A basic toilet brush works great to clean the sides of the pond when algae overgrowth becomes a problem.
6. Clean the pond pump and filter. Clean all of the debris out of the pond pump and filter. It's a good idea to fill a 5 gallon bucket with pond water (or use rainwater) to rinse the pump and filter rather than chlorinated water from the garden hose. Otherwise, the chlorine will kill beneficial bacteria. You can simply fill up a small bucket with pond water and use it to rinse the pump and filter. However, if you're in a hurry, you can just use the hose to give your pond filter a quick rinse.
7. Put everything back in the pond. Now that you're done cleaning the pond, you can put the pump, plants, decorations, and fish back into the water. If you drained the pond to clean it and filled it back up with chlorinated water, be sure to let it sit for at least 24 hours before putting the fish back into it. Chlorinated water will kill your fish and you don't want that to happen.

(Adopted from: https://getbusygardening.com/fish-pond-cleaning/ (April 23, 2019))

Why should we use pond water or rainwater to rinse the pump and filter rather than chlorinated water from the garden hose?