Aquarium tank water change?

Is it a gravel filter that just blows bubbles or an actual filter with a filter pad, etc? If it is a filter with pad, then you can change the water out less often, say 20-25% once or twice a week. Do a 100% water change once a month, at least. If there is really not a filter system in it, then you should perform partial water changes more often and 100% every other week or so. You will notice signs if your water is not changed often enough, your fish will show signs of stress. Be sure to add de-chlorinator to the new water that you are using, even for partial water changes. If the water gets to looking cloudy, it needs to be changed more often, for sure

Take a large bowl that is at least one gallon, but the fish in that with the current water. Then empty the tank clean & refill. Run the filter to 24 hours, the re-introduce the fish to the new clean tank.