the bellow are correct that they should be in a bigger tank... BUT, not necessarily heated. I don't have one in mine since the temperature in the tank is always the right temperature for the fish due to the climate i live in. I just have a filter to keep it clean and mine has been alive for many years now, but that is not the question, so i will answer your question best i can =)
I find using a siphon works well, you can vacuum the gravel whilst the fish is in it, for any ornaments or plants you have in there, u can lift them out using something that has been run through hot water first (boiled so there is no chlorine or anything like that) DO NOT ever put your hands in the tank, it will transfer fungi and other chemicals. Wash anything you take out in boiled water, and you can use something like a toothbrush taken straight out of a packet, do not use anything that has ever seen any other chemicals. I find using a toothbrush to clean the inside of my tank works well also, it doesn't disturb the fish and all is well. Just make sure anything that goes into the tank is sterile and remember to clean 1/4 of the tank every week.
Hope you find some use in that =) good luck