i like to use vinagar and a hard bristle toothbrush it takes a little work but it gets the tank sparkeling just make sure you rinse it out really well, iv had a bad experience with that, but it wasnt my fault my uncle decided to clean my tank and well he knows nothing about fish and took them all out put them in a cut up 2L bottle and washed my take with bleach, rinsed once and put them back in......
it didnt go over too well... i had to start everything all over