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This is the most efficient, and often preferred, way of performing a partial water change, especially in larger tanks. Attach the device directly to your faucet and then insert one of the connected hoses and siphon attachments into your tank. The device will automatically suck out water for you, until you switch it off. Then, flip the switch again and insert the faucet hose to refill the tank. This method is especially useful for those people unable to haul multiple buckets of water around in order to perform regular cleanings. It also minimizes the potential to create a watery mess. Just make sure that the new water that you are adding is close to the original temperature of the tank. Check this before starting the automatic suctioning process. If you don't have an all-in-one system, then you will need to perform the water switch by hand. Start by placing the tube end of your siphon into a bucket. Then, place the siphon tip into the tank's substrate, usually gravel or sand. Repeatedly insert the siphon deep into the sand at an angle, drawing out both debris and tank water. Don't feel as if you have to perfectly clean all of the gravel during each change. In fact, it is better if you section off your tank and clean certain areas of the gravel each time. This lessens the impact of the change for your fish. As you move your siphon around, you will notice that your bucket begins to fill with dirty gravel and murky tank water, this is perfectly normal and what you want. However, don't take it too far. Use your siphon to remove a maximum of 30% of the tank's water. Going beyond this can irreparably alter your tank's chemistry. For example, if you have a 10 gallon tank, it is a good idea to use a 3 gallon bucket for water changes. Then, when the bucket is full, you know that you've removed the right amount of water. Now that your tank is less full, spend a bit of time checking out the status of your tank from an inside view. If you don't fully remove your decorations, then perhaps pick them up and look them over for any damage. Check to make sure that all of your heating and filtration systems seem okay in structure. If you have a tank-side thermometer, make a quick note of the temperature of the water post-drain. Otherwise, it is a good idea to dip a thermometer into the water to get a reading. Then, test the temperature of the clean, treated water that you will add to the tank in a bit. Make sure that the two temperatures match. If not, you may need to give it a some more time before completing the change. Water temperature fluctuations can make fish more susceptible to illness. Make sure to take the tank's temperature again after you've added in the fresh water. At this point, you will want to move the pretreated water from the bucket into the tank. You can do this by pouring the water into a pitcher and then emptying the pitcher into the tank. Or, you can hold the bucket with both hands and empty it directly into the tank. Whichever method you choose, just make sure that the water doesn't rush in super-fast and disturb the gravel and your decorations. Some people like to use their hand, or a plate, to lessen the direct flow of the water into the tank. If you've taken out any of your artificial decorations, you can add them back in either immediately before, or directly after, refilling the tank. You can use this as an opportunity to move items around, or to leave items out entirely for a new look. All of the functioning systems that you unplugged at the start of this process now need to be hooked up again. Make sure your hands are dry and carefully reinstall and restart all of these items. Some types of filers, such as the hang-on ones, will need 1-2 cups of water directly fed into the system before they will start working. Create a designated area where you keep all of your tank cleaning supplies. You will want to allow your buckets, scrubbers, and siphon to air dry before storing. Keeping up with regular storage procedures may keep you from having to buy replacements.

Summary:
Use an automatic water changer. Siphon out any substrate debris with a gravel siphon. Suck out water from the tank. Take a detailed look at your tank's interior. Note the temperature of the remaining water. Re-fill the tank with pretreated water. Replace all decorations and plants. Reconnect the filtration system, heater, and light. Rinse off and store your equipment.