you can use PURE silicone caulk that has NO
antifungal, or antimicrobial agents in it. i think i used GE Type I caulk... but make sure that is the right one (one of them will have the anti-fungal stuff and one wont - Type I or Type II). i used about 2/3 of a regular size tube for my 55 gal
if you are resealing a tank without taking it apart, then i can help you...
make sure you have the tank in a place that will allow you to leave it there for up to a week. you dont want to move it once you start this process. also, the silicone caulk smells like vinegar, so do it in a well ventilated area. unless you like the smell of vinegar.
to reseal the tank, you will need to remove the old silicone. i used a hand held scraping razor. make sure you have lots of blades, as you will wear them out fast when you scrape against the edges of the glass. plus it is a lot easier with a sharp blade. vacuum out all the silicone that you have scraped off.
go over the areas again with the razor to get all the little pieces that were left behind the first time. vacuum again. you may have to scrape a third time, but just keep at it until you get all the silicone off the glass. new silicone will not stick to the old stuff very well.
clean the area with rubbing alcohol to make sure the glass is clean. wipe it dry with a CLEAN paper towel. use masking tape to mask off the area that you dont want to get silicone all over. i put the edge about 3/4 inches from the corners. do this on all of the panes of glass. get all you taping done at the same time. you dont want to have to go back and do it once you start siliconing.
after all the taping is done. clean the area one more time with the rubbing alcohol. dry it with a clean paper towel.
now the fun part. make sure you have a lot of paper towels before you start caulking. run a bead of caulk around the bottom pane of glass. i think i tried for a 1/4 to 3/8 inch bead. immediately smooth the bead with your finger. clean off with the paper towels as needed. try to do this as fast as possible. do the four vertical corners all the way to the top, and smooth them with your finger. i made sure to get all the way to the brace at the top. for all the edges, make sure that you have good contact between the silicone and both pieces of glass.
dont try and re-smooth an area. once you go over it and leave it for a few minutes, the silicone starts to cure, and you will only make it worse by trying to go over it again to smooth it out...
be careful when you are leaning over the tank and smoothing the silicone. if you inhale deeply, you will get a pungent whif of vinegar... not too fun.
remove the tape as soon as possible. if not, your silicone will stick to the tape, and WILL pull off when you remove the tape. let the tank dry for at least 24 hours. the silicone will be good in a few hours, but let it dry for at leats 24 hours. if you can wait, let it dry for 48 hours. then you can fill the tank with tap water to see if there are any leaks.
i have done 3 tanks this way. my main 55 and two 10 gal
tanks. i messed up the 10's becuase i left the tape on too long. and by too long, i mean 30 minutes after i finished the silicone application. so, get it off as soon as possible. while the silicone is still wet.
best of luck... wow... long message