cleaning my tank?

Question
My roomate left his fish at my house, i know anything about them i have been feeding them but now the tank needs to be cleaned ,i want to empty it and putin new marbles and decore how can i do this safely with out hurting the fish?also one of them is a jack demsy i bought it some rosies (little goldfish is what they look like ) it does not seem to want to eat them why ?

Answer
hey.

Some people do think tanks should have a 100% annual cleaning. this is not true.

You should vacuum the tanks every week to get rid of waste and to lessen nitrate and ammonia. You do this with a siphon from a pet store, often 10 dollars.
You want about 40% of the water gone, then add dechlorinated water. To dechlorinate it, buy some water conditioner at a fish store. Its often 1 capful per 10 gallons of water.

For decoration, wash your hands and dry them well, and place the new decorations in. Make sure they are for aquarium use or else they could release harmful substances.

As for the Dempsey, try a variety of foods. If hes not interested in the live food, try bloodworm, cichlid pellets,  flakes, or freeze dried krill.
If he still will not eat after lots of foods are tried, he may be sick.
Remember, feed fish lightly. Fish can go for 1 month without food and should be fed a reasonable amount, like for your dempsey a few flakes a day, 1 or 2 feeders depending on size ( guppies are good feeders), or the ammount of pellets specified on the food can.