2 tubes seems alot but better to go more than less. I would suggest having nice straight taped edges if your planning on doing it for the first time, keep a rag handy an keep your hands nice an clean. leave it 48hours to cure an give it a good couple of washes before using. make sure you clean the surfaces before applying an remember to wipe off excess asap, if any drys an is left it can be removed with a thin blade(try avoid craft knife blades) ive also heard sunlight soap is good for getting it clean, ive always used turps and never had any issues with so called "smudging"
also ive used selleys stuff, imo its way overpriced but it does the job an unless you gona buy a pack of 12/24 its hardly worth attempting to get a better deal.
always read the back of the tube it will say weither its ok for aquarium or not