I did an interior reseal on my 180 that i picked up used. It's not a super difficult job, but it is very time consuming. You have to remove ALL the interior silicone. Silicone does not adhere to cured silicone, so it is imperative that all of it is removed including the residue looking film that tends to stick behind. You'll go through a tooooon of razor blades in this process, so buy a 100 pack at lowes/hd. The overflows for me were the most difficult part as they must be removed to get a complete interior reseal and those puppies are on there TIGHT! Lots of silicone and unlike the edge seams, you have to get between the plastic and glass. I found a guitar string worked best for me at this. thread it through then pull on both ends and it will cut through like butter (hard frozen butter, but you'll get through it)
Afterwards, clean all the corners with alcohol or another solvent. This will ensure there's no oils that will prevent adhesion of the silicone. You'll likely find more silicone remnants in this process.
Now, you can begin the reseal. Pick up a tube of high quality silicone (I recommend Momentiv RTV 108 for strength. I know this silicone is not holding much is any weight, but always better to go stronger than necessary than not....) and load your caulking gun. I went through about 3 tubes on my 180 (i did big ol fat seams on the bottom) tape out the edges you want with painters tape for a clean line later. I cut an old store discount card of some kind to make the shape of the seam. You'll need to work quickly to make sure you get the seams done before the silicone cures but if done right, you'll be able to have one solid piece of silicone once cured. This is easiest if you have 2 people. one person shoots the silicone and the second person shapes it with the trimmed card. (Wear rubber gloves as you will get silicone everywhere and it makes a mess)
Lastly, reinstall the overflow once the other silicone is cured. Let this all cure for a week and then feel free to leak test.
If done right, you'll have a nice new set of seams that should last a long time! It will be nerve wracking on a large tank (or it was for me.....) but as long as the structural seams are still nice and strong, they'll be what holds the tank together. This just protects those.
This took me about 4 hours a day for 3 days to do this. (plus a few painful hand cramps while scraping the silicone.....) IT's a labor intensive job, but not difficult, just tedious.
