You'll need a small gravel vacuum and one or two 5 gallon buckets. Place the bucket on the ground with the tubing running into it, push the gravel vacuum into the water, pump it up and down a few times (I'm sure there are youtube videos that show how to do it) gravity should start the water siphoning out. Remove 30-50% of the water while "vacuuming" the gravel thouroghly. Replace with water that has been treated with water conditioner and is approx the same temperature as the tank water. Pour the new water onto a small plate or flat rock or ornament to avoid disrupting your gravel and plants. If you only have one cory and suffient filtration then you need only do this every other week or so. Keep in mind that 10 gallons is a very very small tank and you can not keep any goldfish and only keep a few other tropicals. Also (I read) that corys need to be kept in groups of three (which will nearly max out your tank) as they are schooling fish. Also keep in mind that corys do best with a p.h. of around 6.5 so they cannot live with certain types of tropicals (and certainly not with goldfish). Do you have a test kit already? How long has your tank been set up? If it has not yet cycled (which takes a month) then do water changes of 50-60% every other day and test the water frequently.