There is a product called ......uh....... GoofOff and another one - comes in a little tiny bottle at the hardware store that is specially made for dissolving the sticky stuff that sticks stickers on things. It will not hurt the fish - for it is on the OUTSIDE.
BUT:
Try taking a wet rag and setting it so that it keeps the stickers very wet. If they are plastic types, take a knife or a fork or some such implement and scratch/scrape the sticker so that you break open the plastic coating so the paper backing is exposed. Gentle scraping will not scratch your tank. You are not trying to remove the sticker - just break open the plastic covering so the paper part can absorb the water.
Then wet the sticker. Use a touch of soapy water if you can and set the cloth so that it keeps the sticker wet for a few hours. The soap is just a wetting agent - not necessary if you do not feel comfortable with it. Do not let the rag dip into the tank or it will wick water out onto the floor!
When the sticker is fully wet, you should be able to rub or scrape off all the paper and plastic portions of it. Try using a spatula - even the non-scratching ones if you prefer - to scrape off all that you can off the tank.
Now you will only have the glue yuck left on the glass. This can be removed with peanut butter. Like the vegetable oil it works to remove the sticky stuff, but, unlike the oil, it is thick and easy to work with. Run the peanut butter onto the sticky and rub. Get out that spatula again and scrape. It will come off. You can clean up the last little bit with peanut butter again. Then wipe the tank clean and shiney. No need to wash.
Ta DA! A Clean Tank!