For most things, yes, but it doesn't always have to be for a week. Normally I put "do a 75% water change and gravel clean every day for a week or until the problem is identified". If we can figure out what the problem is straight away, and it isn't water quality related, then one or two water changes are fine. But sometimes it takes a few days for people to post images and relevant information about the tank, and in these situations, doing daily water changes and gravel cleans gives them something to do and can help the fish by diluting whatever bad things are in the tank.
The big daily water changes and gravel cleaning dilute any ammonia, nitrite, nitrate or other poisons that might be in the water, as well as diluting disease organisms that might be growing in large numbers. This buys you time to find out exactly what is wrong with the fish and then treat it accordingly. The cleaner environment then helps the fish recover quicker. If you need to use medications, they work best in clean tanks with no rotting fish waste or biofilm on anything.
Doing water changes and cleaning the tank and filter gives the fish keeper something to do while they try to find out what is going on. This lets them do something constructive rather than just fret about the fish being sick. It's a practical thing to do that helps the fish and alleviates the stress in the fish keeper. It's a bit like the doctor telling you to go boil some water while they are attending to the woman who is about to give birth in the bedroom. It gives you something to do.
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Over half the problems people have with fish are caused by poor water quality. So doing a big daily water change and gravel clean can help those people straight away and one water change can sometimes fix the issue if it's something like an ammonia, nitrite or nitrate reading. Even if the problem is fixed after one water change, it's still a good idea to do a few more water changes in quick succession (once a day for a few days) to make sure everything is good.
If there is a disease involved, the big daily water changes and gravel cleaning will dilute the number of pathogens in the water. This means there are fewer organisms to infect the fish and make them sick, and the fish can often live longer with the disease because there are fewer things infecting them.
The same deal with cleaning the filter if it hasn't been done in the last 2 weeks. It removes gunk that grows harmful disease organisms. The fewer disease organisms in the tank, the easier it is for the fish to fight off the disease. Some diseases can't be fought off easily and need medication or salt. A clean tank and filter allows salt and medications to work more effectively.
So yes, basic first aid for fish should include cleaning the tank up and doing big daily water changes and gravel cleans until the problem is identified.
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Increasing aeration also helps by maximising the oxygen levels in the water and making it easier for the sick fish to breath. This is akin to people being put on oxygen when having problems breathing. Clean water, a clean tank, and a clean filter will have more oxygen than a dirty tank and filter because there are fewer bacteria and other microbes using the oxygen. This leaves more oxygen for the fish.
This is a sticky at the start of the Emergency section of the forum and it's hoped people read it if they get a sick fish.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
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I’m actually going to save this answer for the future
