Does anyone suggest i scrap off the visible silicone and apply a new seal to the outside(outside of the inside of the tank that is)?
Not sure why you would do this? Does the tank leak? If so I would try to patch the leaking area only. If the tank holds water fine then I don't see why you would want to go to all that trouble to remove the silicone from the glass just to reapply.
Setting the tank up against an outside wall is good as the outside wall is a load bearing wall. A 125 gal
tank i belive is 4 foot long. Standard building practices would dictate 16" joists distance at a maximum so that would mean that the weight would be spread out over three floor joists. If you had the tank for example with the short end toward the wall this would put all the weight over 1 joist or maybe 2 if you was lucky and placed it so the front and back was over a joist. Having the tank within a foot from the wall will also be good as the joists are much more stronger at the ends than in the middle.
Do you have access from the lower level to check out the floor supports? If so you could "double up" the floor joists in the area that your going to put the tank for added support. If you are very unsure I would contact a home improvment contractor and get their opintion.
I would suspect the full weight of the tank when in operation would be close to 1500lbs where 1000lbs of that is the water itself. But if you can get it over 3 floor joists then your effectivly putting 500lbs over each joist. Thats really not alot of weight when you think about the weight of a room full of furniture and people.
I don't think you will run into a problem unless your condo was built out of code.
DISCLAMER - I AM NOT A BUILDING CONTRACTOR