Question On Resealing A Tank

A silicone sealant that is aquarium safe. That's all tank manufacturers use. You must make sure it is fish-safe though - a lot of sealants contain anti-mould chemicals which could kill the fish. Either use a brand designed for aquariums (though they are ususally more expensive) or read the small print on a non-aquarium brand to see if it is safe for aquariums.

They both look OK from the description - they both say they are fish safe - though I haven't used either myself.  I prefer to get such things from a real shop, that way I can read everything written on the tube before I buy it. But that does work out more expensive.

Silicone sealant is all tank manufacturers use. If in doubt use the sealant, wait 48 hours or whatever the tube says for it to cure then fill the tank with water somewhere a leak won't matter. When I get a new tank, I fill it up on the patio first  . Though with the size you are talking about, that could take a fair time to empty afterwards. And I can see why you don't want that much water on your floor - those dimensions work out at 680 litres.