Hello; I have used non-iodized table salt to clean tanks for a lot of years. I make up a paste by ading a small amount of water to some of the salt. I then use a soft sponge or something that will not scratch glass to rub the salt paste on the inside and outside surface of the tank. The salt paste does not scratch glass and is abrasive enough to do a decent job of cleaning. Be sure to use salt that does not contain iodine. I have cleaned algae and gunk from silicone seams this way as well as the glass surface.
I would likely wet the inside of the tank before trying the salt paste. My proceedure with a dirty tank is to place it somewhere, usually outside, and fill it up to check for leaks. No leaks, then siphon the water out and try to rinse as much of the gunk out as possible. One way to siphon most of the water is to have a section of a 2x4 on hand to put under one end of a tank to raise it up (don't try this with water in the tank and I don't have a board under the tank when filling it all the way up.) and send the water to the other end. I can then use a hose to flush stuff toward the siphon. Keeping a siphon going while flushing with a hose would likely make an episode of funny home videoes, but it can be done. I usually have several short sections of board to hold either the hose of the siphon tube in position at the top of a tank.
Buy the time I get the bulk of the gunk rinsed out the stubborn places are thoroughly wet and maybe softer. I then mix up the salt paste. There is often enough water in a corner of the tank to make enough paste. Note- any small pieces of gravel or orther hard bits that you miss have a way of getting into the paste and can scratch the glass, so keep an eye out.
You can rinse the salt paste out right away after the tank is scrubbed or I sometimes allow it to dry to a haze. My thinking is that this may dessicate any disease agents somewhat. Makes me feel better about setting up a healthy tank anyway.
You are likely going to be able to clean the silicone. Even if the silicone does not come super clean, re-sealing a tank with silicone is a big and often messy task. Especially if it does not leak.
